(7) باب فِي الْوِتْرِ قَبْلَ النَّوْمِ
Abu Hurairah said:
My friend (i.e. the Prophet) instructed me to observe three practices that I do not leave while traveling 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.
Reference: Sunan Abi Dawud 1432
In-book reference: Book 8, Hadith 17
English translation: Book 8, Hadith 1427