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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا جَرِيرٌ، عَنْ عَبْدِ الْمَلِكِ، - وَهُوَ ابْنُ عُمَيْرٍ - عَنْ قَزَعَةَ، عَنْ أَبِي سَعِيدٍ، - رضى الله عنه - قَالَ سَمِعْتُ مِنْهُ، حَدِيثًا فَأَعْجَبَنِي فَقُلْتُ لَهُ آنْتَ سَمِعْتَ هَذَا، مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ فَأَقُولُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم مَا لَمْ أَسْمَعْ قَالَ سَمِعْتُهُ يَقُولُ " لاَ يَصْلُحُ الصِّيَامُ فِي يَوْمَيْنِ يَوْمِ الأَضْحَى وَيَوْمِ الْفِطْرِ مِنْ رَمَضَانَ " .
sunnah.com