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As for “the criteria of al-Bukhārī and Muslim,” each transmits from certain narrators whom the other does not use, while they also share other narrators. The hadiths included by both centre on these shared narrators.

One of them may transmit from a narrator through supporting chains and corroborating reports, without relying on him for the principal report. He may transmit from him reports already known to him through other narrators, but not those he alone transmits. He may also omit reports from a trustworthy narrator when he knows that the narrator erred in them. Someone without expertise may therefore suppose that the compilers of the Sahīh collections accept everything that such a narrator transmits as authoritative evidence, but that is not the case.

Recognizing hidden defects in hadith is a noble discipline understood by the leading authorities in the field, such as Yahyā ibn Saʿīd al-Qattān, ʿAlī ibn al-Madīnī, Ahmad ibn Hanbal, al-Bukhārī—the compiler of the Sahīh—al-Dāraqutnī and others. These are branches of knowledge understood by their specialists. And Allah knows best.