From Sahih Muslim (Hadith 2673):
Qaza'a related from Abu Sa'id. He said:I heard from him (Abu Sa'id) a hadith which impressed me, and I said to him: Did you hear it from the Messenger of Allah (ﷺ)? Thereupon he said: (Is it possible) that (I should) say about the Messenger of Allah (ﷺ) that which I have not heard? I heard him saying: It is not proper to fast on two days, Adha and Fitr (at the end) of Ramadan
Reference: Muslim 2673