Sunnah.com reference : Book 2, Hadith 274English translation : Book 2, Hadith 365Arabic reference : Book 2, Hadith 370
Narrated Abu Ayub al-Ansari (RA):
Allah's Messenger (ﷺ) said: "al-Witr prayer is a duty upon every Muslim, so whoever likes to offer it with five Rak'at let him do so and whoever likes to offer it with three let him do so, and whoever likes to offer it with one let him do so."

[Reported by al-Arba'ah except at-Tirmidhi. And Ibn Hibban graded it Sahib (authentic) and an-Nasa'i held that the stronger view is that it is Mawquf (saying of a companion)].
وَعَنْ أَبِي أَيُّوبَ اَلْأَنْصَارِيِّ ‏- رضى الله عنه ‏- أَنَّ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-قَالَ : { اَلْوِتْرُ حَقٌّ عَلَى كُلِّ مُسْلِمٍ , مَنْ أَحَبَّ أَنْ يُوتِرَ بِخَمْسٍ فَلْيَفْعَلْ , وَمَنْ أَحَبَّ أَنْ يُوتِرَ بِثَلَاثٍ فَلْيَفْعَلْ , وَمَنْ أَحَبَّ أَنْ يُوتِرَ بِوَاحِدَةٍ فَلْيَفْعَلْ }  رَوَاهُ اَلْأَرْبَعَةُ إِلَّا اَلتِّرْمِذِيَّ , وَصَحَّحَهُ اِبْنُ حِبَّانَ , وَرَجَّحَ النَّسَائِيُّ وَقْفَهُ .‏  1‏‏1 ‏- صحيح .‏ رواه أبو داود ( 1422 )‏ ، والنسائي ( 3 / 238 )‏ ، وابن ماجه ( 1190 )‏ ، وابن حبان ( 2410 )‏ .‏