(20) باب صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوِتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ
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 754 b
In-book reference: Book 6, Hadith 194
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1649