Musnad Ahmad 25:1088

Yahya ibn Hammad narrated to us, Abu 'Awanah narrated to us, from Abu Bishr, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "By Allah, the Messenger of Allah (peace and blessings of Allah be upon him) never fasted a complete month at all, other than Ramadan. And when he fasted, he would fast until the observer would say: 'By Allah, he will not break his fast.' And he would break his fast when he broke it, until the observer would say: 'By Allah, he will not fast.'"