Musnad Abu Ya'la al-Mawsili 152:108

On the authority of Abd al-Rahman ibn Yazid, he said: Abdullah said, "I did not see the Messenger of Allah (may Allah bless him and grant him peace) perform any prayer except at its time, except for two prayers. I saw him perform the Maghrib and Isha prayers together at Muzdalifah, and he performed the Fajr prayer before its [usual] time, in the darkness of the night."