It is narrated on the authority of Abu Hurayra (may Allah be pleased with him) who said: 

My friend (the Messenger of Allah sallallahu 'alayhi wa salam) directed me to observe FAST FOR THREE DAYS IN EVERY MONTH, to perform two Rak`ah (optional) Duha prayer at forenoon and to perform the Witr prayer before going to bed. 

[Bukhaari,#1124; Muslim, #721]