(20) باب صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوِتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ
Abu Sa'id (al Khudri) reported Allah's Apostle (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.
Reference: Sahih Muslim 754 a
In-book reference: Book 6, Hadith 193
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1648