פירוש על בבא מציעא 66:15

Since from the time of the disciples of Shammai and Hillel, three generations before him [meaning Rebbi], there were great disputes regarding the meanings of the Torah and there arose the possibility of there being two Torahs amongst Israel, due to the oppression of the kingdom [Rome] and the evil decrees passed against Israel. Because of these [troubles] they [the scholars of Israel] were unable to clarify the differing opinions and settle them, until [came] the time of Rebbi. Then did the Lord give favor unto Rebbi in the eyes of the Roman emperor, Antonius, and the troubles subsided, and Rebbi was able to gather all of the scholars of the Land of Israel to him [in Beit She’arim and Tzippori]. Until his [Rebbi's] days, there were no ordered tractates [of the Oral Law] but rather every student studied and reviewed lectures that he heard from the great men and he ascribed to them these teaching s-"this halachah heard from this and this scholar." Now, when they all gathered together [at Rebbi's yeshivah], each of the scholars repeated what he had learned and together they worked to clarify the reasons behind disparate opinions and they settled as to which opinion was to be deemed correct. And then they ordered these opinions and decisions into tractates: the laws of torts by themselves, the laws of levirate marriages by themselves, the laws of the Temple service by themselves, [etc]. And they quoted the opinions and decisions of many scholars anonymously, for Rebbi agreed with their decisions and therefore quoted them anonymously in order to indicate that so is the halachah.