I witnessed Ali and Uthman on the day of Fitr and the day of Nahr. They would perform the prayer, then turn to face the people and admonish them. I heard them both say, "The Messenger of Allah ﷺ forbade fasting on these two days."
حدثنا عبد الله حدثنا محمد بن أبي بكر حدثنا خالد بن الحارث حدثنا ابن أبي ذئب عن سعيد بن عبد الله بن قارظ
شهدت عليا وعثمان رضي الله عنهما في يوم الفطر والنحر يصليان ثم ينصرفان فيذكران الناس فسمعتهما يقولان نهى رسول الله ﷺ عن صوم هذين اليومين
Its isnad is sahih. Muhammad bin Abu Bakr is Muqaddami. Saeed bin Abdullah bin Qariz is Saeed bin Khalid bin Abdullah bin Qariz, attributed to his grandfather, and he is trustworthy. Abu Ubayd is the freedman of Ibn Azhar, and his name is Saad bin Ubayd, whom we have discussed previously at 224. This hadith is among the additions of Abdullah bin Ahmad, see also 282 and 435.