Musnad Ahmad 25:160

Yahya narrated to us, from Shu'bah, Abu Bishr narrated to us, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to fast until we would say he does not break his fast, and he would break his fast until we would say he does not fast. And he did not fast a complete month since he arrived in Medina, except for Ramadan."