'Abdullah bin Shaqeeq Radhiyallahu 'Anhu reports:
"I inquired from 'Aayeshah Radhiyallahu 'Anha regarding the (nafil) saum of Rasulullah Sallallahu 'Alayhi Wasallam. She replied: 'At times Rasulullah Sallallahu 'Alayhi Wasallam fasted continuously. We used to say, he will keep on fasting (this month). At times he did not fast, till we began thinking he will not fast (that month). Rasulullah Sallallahu 'Alayhi Wasallam did not fast for a month after he came to Madinah, besides the fasts of Ramadhaan'".
English reference: Book 42, Hadith 281
Arabic reference: Book 43, Hadith 298