The Book of Prayer - Travellers
6/20. Chapter:
The night prayers are two by two, and Witr is one rakah at the end of the night
Sahih Muslim
وَحَدَّثَنِي إِسْحَاقُ بْنُ مَنْصُورٍ، أَخْبَرَنِي عُبَيْدُ اللَّهِ، عَنْ شَيْبَانَ، عَنْ يَحْيَى، قَالَ أَخْبَرَنِي أَبُو نَضْرَةَ الْعَوَقِيُّ، أَنَّ أَبَا سَعِيدٍ، أَخْبَرَهُمْ أَنَّهُمْ، سَأَلُوا النَّبِيَّ صلى الله عليه وسلم عَنِ الْوِتْرِ فَقَالَ " أَوْتِرُوا قَبْلَ الصُّبْحِ " .
Abu Sa'id reported that they (some of the Companions) of the Prophet (ﷺ) asked the Messenger of Allah (ﷺ) about Witr. He said: Observe Witr before morning.
Reference : Sahih Muslim 754bIn-book reference : Book 6, Hadith 194USC-MSA web (English) reference : Book 4, Hadith 1649