Musnad Ahmad 25:1135

Yahya ibn Adam narrated to us, Mufaddal narrated to us, from Mansur, from Mujahid, from Tawus, from Ibn Abbas, who said: "The Messenger of Allah (peace be upon him) traveled in the year of the Conquest during Ramadan. He fasted until he reached Usfan, then he called for a vessel and drank during the day so that the people could see him. Then he broke his fast until he entered Makkah, and he conquered Makkah during Ramadan." Ibn Abbas said: "The Messenger of Allah (peace be upon him) fasted while traveling and he broke his fast, so whoever wishes may fast, and whoever wishes may break his fast."