The Prophet ﷺ used to perform the witr prayer with three rakahs, reciting "Glorify the name of your Lord, the Most High" [Ala 87:1], "Say, O disbelievers" [Kafirun 109:1], and "Say, He is Allah, the One" [Ikhlas 112:1].
حدثنا إسحاق بن عيسى حدثنا شريك عن أبي إسحاق عن سعيد بن جبير
أن النبي ﷺ كان يوتر بثلاث بسبح اسم ربك الأعلى وقل يا أيها الكافرون وقل هو الله أحد
Its isnad is sahih. It was also reported by Tirmidhi, Nasai, and Ibn Majah, as in al-Muntaqa 1210, and it appears in Tirmidhi 1:341, where its commentator attributed it to Abu Dawud, which is a mistake. The hadith will also come again at 2725, 2726. See also 678.