Musnad Ahmad 25:313

Muhammad ibn Ja'far narrated to me, Shu'bah narrated to us, from Abu Bishr, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) would fast until we would say: 'He does not intend to break his fast,' and he would break his fast until we would say: 'He does not intend to fast.' And he did not fast a month consecutively other than Ramadan since he arrived in Medina."