Sahih Muslim 754
Sahih Muslim · Book 6: The Book of Prayer - Travellers · The night prayers are two by two, and Witr is one rakah at the end of the night
Abu Sa'id reported that they (some of the Companions) of the Prophet (ﷺ) asked the Messenger of Allah (ﷺ) about Witr. He said:
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 " .
Observe Witr before morning.