'Abdullah bin Mas'ud Radiyallahu 'Anhu reports:
"Rasulullah Sallallahu 'Alayhi Wasallam fasted for three days at the beginning of every month, and he very seldom did not fast on Friday". (ES)
English reference (ES): Book 42, Hadith 286 (ES)
Arabic reference (ES): Book 43, Hadith 303 (ES)