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

"Jibrail, peace be upon him, came to the Prophet (saw) when the sun had passed its zenith and said: 'Get up, O Muhammad (SAW), and pray Zuhr when the sun has passed its zenith.' Then he waited until a man's shadow was equal to his height. Then he came to him for Asr and said: 'Get up, O Muhammad (SAW), and pray Asr.' Then he waited until the sunset, then he came to him and said: 'Get up, O Muhammad (SAW), and pray Maghrib.' So he got up and prayed it when the sun had set. Then he waited until the twilight disappeared, then he came to him and said: 'Get up, O Muhammad (SAW), and pray 'Isha',' So he got up and prayed it. Then he came to him when dawn broke and said: 'Get up, O Muhammad (SAW), and pray.' So he got up and prayed Subh. Then he came to him the next day when a man's shadow was equal to his height, and said: 'Get up, O Muhammad (SAW), and pray.' So he prayed Zuhr. Then Jibrail came to him when a man's shadow was equal to twice his length and said: 'Get up, O Muhammad (SAW), and pray.' So he prayed Asr. Then he came to him for Maghrib when the sun set, at exactly the same time as the day before, and said: 'Get up, O Muhammad (SAW), and pray.' So he prayed Maghrib. Then he came to him for Isha when the first third of the night had passed, and said: 'Get up and pray.' So he prayed Isha. Then he came to him for Subh when it had become very bright, and said: 'Get up and pray.' So he prayed Subh. Then he said: 'The time of prayer are between those two (limits).'" (Hasan)