Duwaid bin Nafi' said:" Ibn Shihab informed me, saying: 'Ata bin Yazid narrated to me from Abu Ayyub: That the Prophet (ﷺ) 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 عَمْرُو بْنُ عُثْمَانَ، قَالَ حَدَّثَنَا بَقِيَّةُ، قَالَ حَدَّثَنِي ضُبَارَةُ بْنُ أَبِي السَّلِيلِ، قَالَ حَدَّثَنِي دُوَيْدُ بْنُ نَافِعٍ، قَالَ أَخْبَرَنِي ابْنُ شِهَابٍ، قَالَ حَدَّثَنِي عَطَاءُ بْنُ يَزِيدَ، عَنْ أَبِي أَيُّوبَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " الْوِتْرُ حَقٌّ فَمَنْ شَاءَ أَوْتَرَ بِسَبْعٍ وَمَنْ شَاءَ أَوْتَرَ بِخَمْسٍ وَمَنْ شَاءَ أَوْتَرَ بِثَلاَثٍ وَمَنْ شَاءَ أَوْتَرَ بِوَاحِدَةٍ " .
Nesai
The Book of Qiyam Al-Lail (The Night Prayer) and Voluntary Prayers During the Day • Hadith 1710