Musnad Abu Ya'la al-Mawsili 110:276

Zuhayr narrated to us, Muhammad ibn Ubayd narrated to us, Uthman ibn Hakim narrated to us, he said: I asked Sa'id ibn Jubayr about fasting in Rajab, how do you view it? He said: Ibn Abbas narrated to me that 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.