Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2585
'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 conti- nuously) that we (were inclined to say) that he would not observe fast.