'Aayeshah Radiyallahu 'Anha reports:
"Rasulullah Sallallahu 'Alayhi Wasallam fasted three days of every month. In some months he fasted on Saturdays, Sundays and Mondays, and in some months he fasted on Tuesdays, Wednesdays and Thursdays".
English reference: Book 42, Hadith 289
Arabic reference: Book 43, Hadith 306