Sahih Al Bukhari Hadith Number 1981

Narrated Abu Huraira: My friend (the Prophet (peace be upon him) ) advised me to observe three things: (1) to fast three days a month; (2) to pray two rak`at of Duha prayer (forenoon prayer); and (3) to pray witr before sleeping.

30  Fasting
Hadith number 1981
Volume no 3
Chapter: 60 (To fast the 13th, 14th and 15th of the lunar months.)