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

A man asked the Messenger of Allah (saw) about the times of prayer. He said: 'Pray with me.' So he prayed Zuhr when the sun had passed its zenith, Asr when the shadow of a thing was equal to its height, Maghrib when the sun had set and 'Isha' when the twilight had disappeared." He said: "Then he prayed Zuhr when the shadow of a man was equal in length to his height, Asr when the length of a man's shadow was twice his height, and Maghrib just before the twilight disappeared." (One of the narrators) Abdullah bin Al-Harith said: "Then he said: 'With regard to 'Isha' I think it is up to one-third of the night.'"* (Hasan) *The speaker there is Thawr, who narrated it from Ata' from Jabir.