Sahih Muslim Book 6, Hadith Number 2585.

Chapter : The fasts observed by the Apostle of Allah (may peace be upon him) besides Ramadhan.

‘Uthman b. Hakim al-Ansari said: I asked Sa’id b. Jubair about fasting In Rajab, and we were then passing through the month of Rajab, whereupon he said: I heard Ibn ‘Abbas (Allah be pleased with both of them) as saying: The Messenger of Allah (may peace be upon him) used to observe fast (so continuously) that we (were inclined) to say that he would not break (them) and did not observe them so continuously) that we (were inclined to say) that he would not observe fast.