(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. (ES)
Reference (ES): Sunan Abi Dawud 1432 (ES)
In-book reference (ES): Book 8, Hadith 17 (ES)
English translation (ES): Book 8, Hadith 1427 (ES)