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
← Previous 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 Next Hadith → "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