Ibn 'Abbaas Radiyallahu 'Anhu relates:
"Rasulullah Sallallahu 'Alayhi Wasallam fasted the major portion of the month at times, till we thought that he did not intend ending the fasts. In some months he did not fast, we began to think he would not fast now. Besides Ramadaan he did not fast for a full month".
English reference: Book 42, Hadith 283
Arabic reference: Book 43, Hadith 300