Abu Dawud Book 2, Hadith Number 1428.

Chapter : On observing the witr before sleeping.

Narated By Abu al-Darda : My friend (i.e. the Prophet) instructed me to observe three practices that I never leave: he instructed me to fast three days every month and not to sleep but after observing the witr, and to observe the supererogatory prayer in the forenoon while travelling and while resident.