Volume 3 · Book 31 · Hadith 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.
— Sahih Bukhari, Volume 3, Book 31: Fasting, Hadith #202