"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
← Previous Hadith he asked Aishah about the Witr of the Prophet. She said: "He would perform Witr during all of the night; (either) its beginning, its middle, or its end. So when he died, his Witr ended, during the approachof As-Sahar."

Jami` at-Tirmidhi Book 3, Hadith 5 Next Hadith → "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