Sahih Al Bukhari Hadith Number 1178

Narrated Abu Huraira: My friend (the Prophet) advised me to do three things and I shall not leave them till I die, these are: To fast three days every month, to offer the Duha prayer, and to offer witr before sleeping.

19  Prayer at Night (Tahajjud)
Hadith number 1178
Volume no 2
Chapter: 33 (To offer Salat-ud-Duha when not traveling)