Musnad Ahmad 3:29

Abdullah narrated to us, Muhammad ibn Abi Bakr narrated to us, Khalid ibn al-Harith narrated to us, Ibn Abi Dhi’b narrated to us, from Sa’id ibn Abdullah ibn Qarith, from Abu Ubayd, who said: I witnessed Ali and Uthman, may Allah be pleased with them both, on the day of al-Fitr and the day of al-Nahr, praying, then departing, then reminding the people. I heard them both saying: The Messenger of Allah, may Allah bless him and grant him peace, forbade fasting on these two days.