Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

"The Messenger of Allah (saw) used to pray Zuhr when the sun passed its zenith, and he would pray Asr between these two prayers; and he would pray Maghrib when the sun had set, and he used to pray Isha when the twilight had disappeared," then he said straight after that: "And he would pray Fajr when a man could see clearly." (Sahih)