Abu Dawud Book 2, Hadith Number 1427.
Chapter : On observing the witr before sleeping.
Narated By Abu Hurairah : My friend (i.e. the Prophet) instructed me to observe three practices that I do not leave while travelling nor while resident; to pray two rak’ahs in the forenoon, to fast three days every month and not to sleep but after observing the witr.