Commentary for Avodah Zarah 71:11
אמר רב משרשיא מה טעם הואיל ופשט איסורו ברוב ישראל שמן לא פשט איסורו ברוב ישראל דאמר רבי שמואל בר אבא אמר רבי יוחנן ישבו רבותינו ובדקו על שמן שלא פשט איסורו ברוב ישראל וסמכו רבותינו על דברי רשב"ג ועל דברי רבי אלעזר בר צדוק שהיו אומרים אין גוזרין גזירה על הצבור אא"כ רוב צבור יכולין לעמוד בה דאמר רב אדא בר אהבה מאי קרא
R. Mesharsheya said: The reason [that these eighteen things form an exception] is because their prohibition has spread among the large majority of Israelites, but the prohibition concerning oil did not spread among the majority of Israel; for R. Shmuel b. Abba said in the name of R. Yohanan: Our masters sat and investigated [the use of Gentile] oil [and found] that its prohibition had not spread among the large majority of Israelites; they accordingly relied upon the statement of Rabban Shimon b. Gamaliel and R. Eliezer b. Zadok who declared: We make no decree upon the community unless the majority are able to abide by it. R. Adda b. Ahavah said: What Scriptural verse supports this rule?
Daf Shevui to Avodah Zarah
This is a powerful statement concerning one source of Jewish authority. Jewish authority stems not just from what rabbis tell people to do, or from what the books tell people to do, but also from what practices people accept upon themselves. Put another way, people are obligated to accept certain practices simply because other people are practicing in that way.