"The night prayer of Allah's Messenger was thirteen Rak'ah, five of which were his Witr, not sitting in any of them except at the end of them. When the Mu'adh-dhin called the Adhan he would stand to perform two light (Rak'ah)."
Jami` at-Tirmidhi Book 3, Hadith 7
← Previous Hadith "The Prophet would perform Witr with thirteen . When he was older and became weak he performed Witr with seven."

Jami` at-Tirmidhi Book 3, Hadith 6 Next Hadith → "Allah's Messenger would perform Al-Witr with three, reciting nine Surah from the Mufassal in them, reciting three Surah in each Rak'ah with Say: "Allah is One." At the end of them."Muhammad bin Sirin said: "They would perform Al-Witr with five, with three, and with one Rak'ah, and they considered all of that to be good."

Jami` at-Tirmidhi Book 3, Hadith 8