'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". (DE)
English reference (DE): Book 42, Hadith 286 (DE)
Arabic reference (DE): Book 43, Hadith 303 (DE)