Musnad Abu Ya'la al-Mawsili 42:169

Uqbah ibn Mukram narrated to us, Yunus ibn Bukayr narrated to us, Muhammad ibn Ubayd Allah narrated to us, from Atiyyah, from Abu Sa’id, that the Messenger of Allah (may Allah bless him and grant him peace) said: "I forbid you from fasting on two days: al-Fitr and al-Adha."