Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2583
Ibn Abbas (Allah be pleased with both of them) reported: The Messenger of Allah (may peace be upon him) did not fast throughout any month except during ramadan. And when he observed fast (he fasted so continuously) that one would say that he would not break (them) and when he Abandoned, he abandoned (so continuously) that one would say: By Allah, perhaps he would never fast.