Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 202
Narrated Abu Huraira:
My friend (the Prophet ) advised me to observe three things:
(1) to fast three days a month;
(2) to pray two Rakat of Duha prayer (fore-noon prayer); and
(3) to pray Witr before sleeping.