Sunan At-Tirmidhi

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]