Commendations offered by the scholars for the narrators whose reports Muslim includes in his Ṣaḥīḥ—together with the reservations voiced by some ḥadīth masters about a few of them and the answer to those reservations:

Al-Ḥāfiẓ Ibn Ḥajar states in his commentary on Nukhbat al-Fikar: “With regard to the narrators of the two Ṣaḥīḥ collections—that is, those of al-Bukhārī and Muslim—it has been unanimously agreed that they are deemed upright by necessary implication, and their narrations therefore take precedence over the narrations of others. This is a principle from which one departs only on the basis of compelling evidence.”
He adds in the introduction to Fatḥ al-Bārī: “Shaykh Abū al-Ḥasan al-Maqdisī used to say of any man from whom a ḥadīth is cited in the Ṣaḥīḥ: ‘He has crossed the bridge,’ meaning that whatever has been said against him is not to be heeded.”

Some critics have spoken against certain narrators whose ḥadīths Muslim transmitted—one hundred and sixty in total—but such criticism neither undermines his Ṣaḥīḥ nor detracts from its standing, for several reasons.

First: the criticism may be of no real consequence.
Al-Khaṭīb al-Baghdādī, as quoted in the introduction to al-Nawawī’s commentary on Ṣaḥīḥ Muslim, says: “Whenever al-Bukhārī or Muslim relies on a narrator who has been criticized by others, that reliance is based on the fact that no substantiated, consequential criticism—whose grounds are clearly explained—has been proven against him.”
Al-Dhahabī writes in a separate treatise he composed about trustworthy narrators who have nevertheless been criticized: “In my work al-Mīzān I have listed a great many reliable transmitters whose reports were accepted by al-Bukhārī, Muslim, and others, simply because each man’s name appears in the books of criticism. I did not mention them because I consider them weak; rather, it is so that this point be known. Time and again I come across a sound narrator who has been faulted by someone whose opinion carries no weight …” and so on—may Allah have mercy on him.

Second: if the criticism is in fact consequential, Muslim’s inclusion of that narrator can be interpreted as follows:

1- The report may occur only in corroborating chains (mutābaʿāt and shawāhid), not in the primary isnād. Al-Nawawī, in the introduction to his commentary on Ṣaḥīḥ Muslim, quoting Ibn al-Ṣalāḥ, writes: “He may first cite the ḥadīth with a pristine chain whose transmitters are all trustworthy and treat that as the basic route, then follow it with another chain—or several chains—that contain some weak narrators, the purpose being merely to reinforce the report through corroboration.”