"Aayeshah Radiyallahu 'Anha says:
"I did not observe Rasulullah Sallallahu 'Alayhi Wasallam fast for more days in any month (excluding Ramadaan) other than Sha'baan. He fasted for the major part of the month, and nearly fasted for the full month"
English reference: Book 42, Hadith 285
Arabic reference: Book 43, Hadith 302