Musnad Ahmad 25:592

Surayj ibn al-Nu‘man narrated to us, Abu ‘Awanah narrated to us, from Abu Bishr, from Sa‘id ibn Jubayr, from Ibn ‘Abbas, who said: "The Messenger of Allah (may Allah bless him and grant him peace) never fasted a complete month at all, other than Ramadan. And he would fast when he fasted, until the observer would say: 'By Allah, he will not break his fast.' And he would break his fast when he broke his fast, until the observer would say: 'By Allah, he will not fast.'"