صَحِيحُ مُسْلِمٍ
Sahih Muslim 754 a
The Book of Prayer - Travellers
The night prayers are two by two, and Witr is one rakah at the end of the night
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدُ الأَعْلَى بْنُ عَبْدِ الأَعْلَى، عَنْ مَعْمَرٍ، عَنْ يَحْيَى بْنِ أَبِي كَثِيرٍ، عَنْ أَبِي نَضْرَةَ، عَنْ أَبِي سَعِيدٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ أَوْتِرُوا قَبْلَ أَنْ تُصْبِحُوا .
Abu Sa'id (al Khudri) reported Allah's Messenger (ﷺ) as saying: Observe Witr prayer before it is morning. Abu Sa'id reported that they (the Prophet's Companions) asked the Messenger of Allah (ﷺ) about Witr (prayer). (In reply to their inquiry) he said: Observe Witr prayer before it is morning.
Sahih Muslim, 754 aSahihصحيح