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

that Jibrail came to the Prophet (saw) to teach him the times of prayer. Jibrail went forward, with the Messenger of Allah (saw) behind him and the people behind the Messenger of Allah (saw), and he prayed Zuhr when the sun had passed its zenith. Then he came to him when the shadow of a person was equal to his height, and did as he had done before; Jibrail went forward, with the Messenger of Allah (saw) behind him and the people behind the Messenger of Allah (saw), and he prayed Asr. Then Jibrail came to him when the sun had set; Jibrail went forward, with the Messenger of Allah (saw) behind him and the people behind the Messenger of Allah (saw), and he prayed Maghrib. Then he came to him when the twilight had disappeared; Jibrail went forward, with the Messenger of Allah (saw) behind him and the people behind the Messenger of Allah (saw), and he prayed Isha. Then he came to him when dawn broke; Jibrail went forward, with the Messenger of Allah (saw) behind him and the people behind the Messenger of Allah (saw), and he prayed Al-Ghadah.* Then he came to him on the second day when a man's shadow was equal to his height, and did as he had done the day before, he prayed Zuhr. Then he came to him when the shadow of a man was twice his height, and did what he had done the day before, and prayed Asr. Then he came to him when the sun had set and did what he had done the day before, and prayed Maghrib. Then we slept and got up, and slept and got up again. Then he came to him and did what he had done the day before and prayed Isha. Then he came to him when the (light of) dawn was spread (on the horizon)** and the stars were still clear in the sky, and he did the same as he had done the day before, and prayed Al-Ghadah. Then he said: 'The time between these two is the time for prayer.'" (Sahih) *Meaning Fajr, the morning prayer. **The Fajr prayer was elongated because the Prophet (saw) recited at length during the prayer, so that it ended just before sunrise. That defined the end of the time for Fajr, as the beginning of the time was defined by the moment when he started the first Rakah.