Musnad Ahmad 2:59

Abu Sa'id narrated to us, Ibn Lahi'ah narrated to us, Bukayr narrated to us, from Sa'id ibn al-Musayyib, from Umar, who said: We went on a military expedition with the Messenger of Allah, may Allah bless him and grant him peace, during Ramadan, and the Conquest [of Mecca] was in Ramadan, so we broke our fast during both of them.