´Anas bin Sirin narrated that :` 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 [meaning that they were light]."
Hadith Referenceسنن ترمذي / أبواب الوتر / 461
Hadith Gradingالألبانی:صحيح، ابن ماجة (1144 و 1318)