Sunan an-Nasa'i, Volume 2, Book 20, Hadith 1712 (DarusSalam) ⓘ

Al-Awza'I said: "Az-Zuhri narrated to me, he said: 'Ata bin Yazid, from Abu Ayyub: The Messenger of Allah said: 'Witr is a duty, and whoever wants to pray witr with seven (rak'ahs), let him do so; whoever wants to pray witr with five, let him do so, whoever wants to pray witr with three, let him do so; and whoever wants to pray witr with one, let him do so.'"

أَخْبَرَنَا الْعَبَّاسُ بْنُ الْوَلِيدِ بْنِ مَزِيدٍ، قَالَ أَخْبَرَنِي أَبِي قَالَ، حَدَّثَنَا الأَوْزَاعِيُّ، قَالَ حَدَّثَنِي الزُّهْرِيُّ، قَالَ حَدَّثَنَا عَطَاءُ بْنُ يَزِيدَ، عَنْ أَبِي أَيُّوبَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ الْوِتْرُ حَقٌّ فَمَنْ شَاءَ أَوْتَرَ بِخَمْسٍ وَمَنْ شَاءَ أَوْتَرَ بِثَلاَثٍ وَمَنْ شَاءَ أَوْتَرَ بِوَاحِدَةٍ ‏" ‏ ‏.‏