Like Imam Bukhari too, he wrote a good number of books and treatises on hadith, and on related subjects. Ibn anl-Nadeem mentions five books by him on the subject. Haajee Khaleefah adds the names of many other works by him in the same field. In his Sahih he examined a third of a million ahadith, from which he selected only about four thousand, which the hadith scholars unanimously regarded as sound.
Similar to Imam Bukhari, Imam Muslim regarded a hadith as Sahih only when it had been handed down to him through a continuous isnaad of known and reliable authorities, was compatible with other material established in this way, and was free from various types of deficiency. He adopted a threefold classification of ahadith.
Firstly, there were those which had been related by narrators who were straightforward and steadfast in their narrations, did not differ much in them from other reliable narrators, and did not commit any palpable confusion in their reports.
Secondly, there were traditions whose narrators were not distinguished for their retentive memory and steadfastness in narrations.
Thirdly, there were the ahadith narrated on the authority of people whom all or most hadith scholars declared were of questionable reliability. According to Imam Muslim, the first group makes up the bulk of his book; the second is included as corroborative of the first, while the third is entirely rejected.
Imam Muslim strictly observed many principles of the science of hadith which had been to some extent neglected by Imam Bukhari. He draws a distinction between the terms akhbaranaa and haddathanaa, and always uses the former in connection with the traditions which had been recited to him by his own teachers, assigning the latter to what he had in turn read out to them. He was more strict and consistent than Imam Bukhari in pointing out the differences between the narrations of the various narrators, and in stating their character and other particulars. He also showed greater acumen in the arrangement of traditions and their asaaneed in his work, and in presenting the different versions of a single tradition in one place. He added a long introduction, in which he explained some of the principles which he had followed in the choice of materials for his book; and which should be followed in accepting and relating traditions.
Upon completing his Sahih, Imam Muslim presented it to Abu Zar’ah of Rayy, a hadith scholar of great repute, for his comments. Abu Zar’ah inspected it closely, and Imam Muslim deleted everything which he thought was defective, and retained only such traditions as were declared by him to be genuine.
Thus carefully compiled by Imam Muslim, and proof-read by Abu Zar’ah the Sahih has been acclaimed as the most authentic collection of traditions after that of Sahih Bukhari, and superior to the latter in the details of its arrangement. Some hadith scholars hold it to be superior to the work of Shahi Bukhari in every respect.