Sahih Muslim , Book 6, Hadith 193 (In-Book) ⓘ

Abu Sa'id (al Khudri) reported Allah's Apostle as saying: Observe Witr prayer before it is morning. Abu Sa'id reported that they (the Prophet's Companions) asked the Messenger of Allah about Witr (prayer). (In reply to their inquiry) he said: Observe Witr prayer before it is morning.

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 ‏" ‏ ‏.‏