From the earliest days of Islam, oral accounts of the deeds and sayings of the Prophet Muhammad (peace be upon him) circulated amongst Muslims. Sometime after his death, scholars such as Bukhari undertook a major effort to collect such sayings and to validate them.
The process of collection and validation is explained in detail in "A Textbook of Hadith Studies: Authenticity, Compilation, Classification and Criticism of Hadith" by Mohammad Hashim Kamali. Most of the validation effort concentrated on the “chain of transmission” which records how the oral information was passed down from person to person before it reached the hadith collector.
There was much less emphasis on assessing the actual text of the hadith to consider whether it was likely to be something that the Prophet (pbuh) might have said. This is discussed in "Authentication of Hadith – Redefining the Criteria" by Israr Ahmad Khan.
One unfortunate side-effect of the collection and validation effort of scholars such as Bukhari is that some Muslims think that all the hard work of validation has been done by these historic hadith collectors. The attitude is that if a hadith is in a collection such as Bukhari or Muslim, then automatically it must be a “sahih hadith” i.e. a “sound or authentic hadith.” Indeed the collections of Bukhari and Muslim are referred to as "sahih collections".
The Muslims who hold such a naive belief are not the ones who have been trained in hadith studies.
Properly trained hadith scholars (such as the authors of the books mentioned above) are well aware that the hadith in collections such as Bukhari vary from highly reliable to relatively unreliable. Instead it is Muslims who have been brought up to believe that they must accept what they are told without critical thinking who are prone to believe that if a hadith is in Bukhari, it must be accurate and true.
My objective is to demonstrate with specific examples that Bukhari and Muslim contain hadith which are not reliable.
The easiest way to do this is to present multiple hadith about the same circumstances which contradict each other. By definition those hadith cannot all be true.
It is not necessary for me to decide which of those hadith are right and which are wrong. The key point is that since they contradict, they cannot all be true, although it is logically possible that they could all be wrong.
I am indebted to the work of Dr Mohammad Omar Farooq, “Islamic Law and the Use and Abuse of Hadith” and Maulana Mohammad Akram Khan as sources for hadith to consider. However every hadith cited below has been checked by me personally to the hadith collections of Bukhari and Muslim which are freely available at the International Islamic University of Malaysia website and the text copied from there to avoid the risk that the authors mentioned earlier in this paragraph might have made a mistake.
Below are some specific circumstances and related hadith. After setting out the hadith, I explain the contradictions between the hadith quoted. It is logically impossible for all of the hadith quoted to be reliable since they contradict.
Obviously the citations below do not purport to be a comprehensive listing of all hadith in Bukhari or Muslim which are contradictory or which may be unreliable for other reasons.
There are a number of hadith in Bukhari and Muslim on this subject.
Bukhari Volume 4, Book 56, Number 747:
Narrated Rabia bin Abi Abdur-Rahman:
I heard Anas bin Malik describing the Prophet saying, "He was of medium height amongst the people, neither tall nor short; he had a rosy color, neither absolutely white nor deep brown; his hair was neither completely curly nor quite lank. Divine Inspiration was revealed to him when he was forty years old. He stayed ten years in Mecca receiving the Divine Inspiration, and stayed in Medina for ten more years. When he expired, he had scarcely twenty white hairs in his head and beard." Rabi'a said, "I saw some of his hairs and it was red. When I asked about that, I was told that it turned red because of scent."
Bukhari Volume 4, Book 56, Number 748:
Allah's Apostle was neither very tall nor short, neither absolutely white nor deep brown. His hair was neither curly nor lank. Allah sent him (as an Apostle) when he was forty years old. Afterwards he resided in Mecca for ten years and in Medina for ten more years. When Allah took him unto Him, there was scarcely twenty white hairs in his head and beard.
Bukhari Volume 5, Book 58, Number 190:
Narrated Ibn 'Abbas:
Allah's Apostle was inspired Divinely at the age of forty. Then he stayed in Mecca for thirteen years, and then was ordered to migrate, and he migrated to Medina and stayed there for ten years and then died.
Bukhari Volume 5, Book 58, Number 242:
Narrated Ibn Abbas:
Allah's Apostle started receiving the Divine Inspiration at the age of forty. Then he stayed in Mecca for thirteen years, receiving the Divine Revelation. Then he was ordered to migrate and he lived as an Emigrant for ten years and then died at the age of sixty-three (years).
Bukhari Volume 5, Book 59, Number 741:
Narrated Aisha and Ibn 'Abbas:
The Prophet stayed for ten years in Mecca with the Qur'an being revealed to him and he stayed in Medina for ten years.'
Bukhari Volume 5, Book 59, Number 742:
Allah's Apostle died when he was sixty-three years of age.
Muslim Book 030, Number 5794:
Anas b. Malik reported that Allah's Messenger (may peace be upon him) was neither very conspicuously tall nor short-statured, and his color was neither glaringly white nor brown; his hair was neither very curly nor very straight; Allah commissioned him (as a Prophet) when he had reached the age of forty years, and he stayed in Mecca for ten years and for ten years in Medina; Allah took him away when he had just reached the age of sixty, and there had not been twenty white hair in his head and beard.
Muslim Book 030, Number 5805:
'Ammar, the freed slave of Banu Hashim, reported: I asked Ibn 'Abbas how old was he when death overtook the Messenger of Allah (may peace be upon him). He said: I little know that such a thing is not known to a man like you who belong to his people. He said: I asked people about it but they differed with me, and I liked to know your opinion about it. He said: Do you know counting? He said: Yes. He then said: Bear this in mind very well that he was commissioned (as a Prophet) at the age of forty, and he stayed in Mecca for fifteen years; sometime in peace and sometime in dread, and (lived) for ten years after his migration to Medina.
Bukhari 4:56:747 and 4:56:748 - He was 40+10+10=60 years old.
Bukhari 5:58:242 and 5:59:742 – He was 63 years old.
Bukhari 4:56:747 and 4:56:748 – He stayed in Mecca for 10 years.
Bukhari 5:58:190 and 5:59:741 – He stayed in Mecca for 13 years.
Muslim 030:5794 – He was 60.
Muslim 030:5805 – He was 40+15+10=65 years old.
In the Quran God prohibits something called “riba” in Arabic. A number of hadith elaborate on what constitutes riba.
Bukhari Volume 3, Book 34, Number 344:
Narrated Az-Zuhri from Malik bin Aus:
that the latter said, "Who has change?" Talha said, "I (will have change) when our store-keeper comes from the forest."
Narrated 'Umar bin Al-Khattab: Allah's Apostle said, "The bartering of gold for silver is Riba, (usury), except if it is from hand to hand and equal in amount, and wheat grain for wheat grain is usury except if it is form hand to hand and equal in amount, and dates for dates is usury except if it is from hand to hand and equal in amount, and barley for barley is usury except if it is from hand to hand and equal in amount."
Bukhari Volume 3, Book 34, Number 388:
Narrated Abdur-Rahman bin Abu Bakra:
that his father said, "The Prophet forbade the selling of gold for gold and silver for silver except if they are equivalent in weight, and allowed us to sell gold for silver and vice versa as we wished."
In Bukhari 3:34:344 above gold cannot be bartered for silver “except if it is from hand to hand and equal in amount.”
In Bukhari 3:34:388 there are no restrictions on how gold is sold for silver.
Many of the principles of Islamic commercial law are derived from analysis of hadith. There are a number of hadith regarding one particular transaction involving a camel. Only a few of them are reproduced below.
Muslim Book 010, Number 3886:
Jabir b. 'Abdullah (Allah be pleased with them) reported that he was travelling on his camel which had grown jaded, and he decided to let it off. When Allah's Apostle (may peace be upon him) met him and prayed for him and struck it, so it trotted as it had never trotted before. He said: Sell it to me for an 'uqaya. I said: No. He again said: Sell it to me. So I sold it to him for an 'uqaya, but made the stipulation that I should be allowed to ride back to my family. Then when I came to (my place) I took the camel to him and he paid me its price in ready money. I then went back and he sent: (someone) behind me (and as I came) he said: Do you see that I asked you to reduce price for buying your camel. Take your camel and your coins; these are yours.
Muslim Book 010, Number 3891:
Jabir (Allah be pleased with him) reported: My camel had grown tired as Allah's Messenger (may peace be upon him) came to me. He goaded it and it began to jump. After that I tried to restrain its rein so that I could listen to his (Prophet's) words, but I could not do that. Allah's Apostle (may peace be upon him) met me and said: Sell it to me, and I sold it for five 'uqiyas. I said: On the condition that I may use it as a ride (for going back) to Medina. He (the Holy Prophet) said: Well, you may use it as a ride up till Medina. When I came to Medina I handed over that to him and he made an addition of an uqiya (to that amount which had been agreed upon) and then presented that (camel) to me.
Muslim Book 010, Number 3893:
Jabir b. 'Abdullah (Allah be pleased with them) reported: Allah's Messenger (may peace be upon him) bought a camel from me for two 'uqiyas and a dirham or two dirhams. As he reached Sirar (a village near Medina), he commanded a cow to be slaughtered and it was slaughtered, and they ate of that, and as he (the Holy Prophet) reached Medina he ordered me to go to the mosque and offer two rak'ahs of prayer, and he measured for me the price of the camel and even made an excess payment to me.
Muslim Book 010, Number 3895:
Jabir (Allah be pleased with him) reported that Allah's Apostle (may peace be upon him) said to him: I have taken your camel for four dinars, and you may ride upon it to Medina.
Muslim 010:3886 – one uqiya
Muslim 010:3891 – five uqiyas
Muslim 010:3893 – two uqiyas and a dirham or two dirhams
Muslim 010:3895 – four dinars
The above examples were chosen to illustrate contradictions that are clear and undisputable. Bukhari and Muslim respectively must have chosen to include the hadiths within their collections, despite their being contradictory, because they saw no reason to prefer some variants over others.
There are of course other hadith which are contradicted by the Quran or contradicted by history, as discussed in the book "Authentication of Hadith – Redefining the Criteria" by Israr Ahmad Khan. However those failings are less immediately obvious, and some may choose to dispute them, whereas the internal contradictions listed above are immediate and impossible to dispute.
The contradictions demonstrate that it is simply impossible to regard all hadith as 100% reliable, which is the naive belief sometimes put forward by Muslims who lack understanding of the way that hadith scholars classify hadith on a scale of reliability.
I do not share the approach of Quranists who disregard all hadith on the grounds that the reliability of hadith cannot be established indisputably. To do so isolates one from a great deal of historical and religious information that has some value, provided it is approached in the spirit of critical enquiry used by most hadith scholars. While regarding all hadith as 100% reliable is simply naive, choosing to ignore all hadith completely is in my view going to the opposite extreme.