Ibn Khallikan
Ibn Khallikan could find no report of Muslim's date of birth, or age at death, by any of the "ḥuffāẓ" (hadith masters), except their agreement that he was born after 200 AH (815/816). Ibn Khallikan cites Ibn al-Salah, who cites Ibn al-Bayyiʿ's "Kitab ʿUlama al-Amsar", in the claim that Muslim was 55 years old when he died on 25 Rajab, 261 AH (May 875) and therefore his year of birth must have been 206 AH (821/822).