Al-Bukhārī’s Criteria for Inclusion in his Ṣaḥīḥ
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Al-Bukhārī’s specific condition:
Al-Ḥāfiẓ Ibn Ḥajar relates in the introduction to Fatḥ al-Bārī, with his own chain of transmission to the ḥāfiẓ Abū ’l-Faḍl Ibn Ṭāhir al-Maqdisī, that the latter said:

“Al-Bukhārī’s requirement was to record only those traditions whose transmitters are unanimously deemed trustworthy all the way up to the well-known Companion, with no disagreement among the reliable authorities, and whose chain is fully connected, without any break. If that Companion is reported from by two transmitters or more, so much the better; but if there is only one transmitter and the route to him is sound, that suffices.”

This statement, cited by Ibn Ḥajar in his introduction, is affirmed almost verbatim by al-Maqdisī himself at the beginning of his work Shurūṭ al-A’immat al-Sittah. Ibn Ḥajar further remarks, both in the introduction to Fatḥ al-Bārī and in his commentary on Nukhabat al-Fikar, while arguing for the superiority of al-Bukhārī’s Ṣaḥīḥ over Ṣaḥīḥ Muslim:

“As for its advantage in terms of an unbroken chain, that is because al-Bukhārī stipulates that every transmitter must be proven to have met the one from whom he narrates, even if only once, whereas Muslim is content with mere contemporaneity.”

He also states in Sharḥ al-Nukhbah, while listing the ranks of sound ḥadith: “Next in order of preference, from the standpoint of greatest authenticity, are those traditions that meet the joint requirements of both of them, for what is meant by that is their respective transmitters together with the other conditions of soundness …”