"Allah's Messenger would recite during Al-Witr: "Glorify the Name of your Lord the Most High' and, 'Say: O you disbelievers!' and, 'Say: Allah is One,' in each Rak'ah."
Jami` at-Tirmidhi Book 3, Hadith 10
← Previous Hadith he asked Ibn Umar about the length of the two Rak'ah (before) Al-Fajr. He said: "The Prophet would pray two and two during the night, and he would perform Al-Witr with one Rak'ah. And he would pray two Rak'ah while he was hearing the Adhan ."

Jami` at-Tirmidhi Book 3, Hadith 9 Next Hadith → "I asked Aishah about what (recitation) Allah's Messenger would perform Al-Witr with. She said: 'In the first he would recite: Glorify the Name of your Lord the Most High, in the second: Say: O you disbelievers!, and in the third: Say: Allah is One" and, Al-Mu'awwidhatain.'"

Jami` at-Tirmidhi Book 3, Hadith 11