Musnad Ahmad 25:1150

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 will not break his fast, and he would break his fast until we would say: He will not fast."