Musnad Ahmad 33:423

Yahya ibn Adam narrated to us, Ibn al-Mubarak narrated to us, from Husayn ibn Ali, who said: Wahb ibn Kaysan narrated to me, from Jabir ibn Abdullah—and he is the Ansari—that Gabriel came to the Prophet, may Allah bless him and grant him peace, and said: "Stand and pray." So he prayed Dhuhr when the sun had passed the meridian. Then he came to him for Asr and said: "Stand and pray." So he prayed Asr when the shadow of everything became equal to it—or he said: when its shadow became like it. Then he came to him for Maghrib and said: "Stand and pray." So he prayed when the sun had set. Then he came to him for Isha and said: "Stand and pray." So he prayed when the twilight had disappeared. Then he came to him for Fajr and said: "Stand and pray." So he prayed when the dawn shone—or he said: when the dawn broke. Then he came to him the next day for Dhuhr and said: "Stand and pray." So he prayed Dhuhr when the shadow of everything became equal to it. Then he came to him for Asr and said: "Stand and pray." So he prayed Asr when the shadow of everything became twice its length. Then he came to him for Maghrib at the same time, not deviating from it. Then he came for Isha when half the night—or he said: a third of the night—had passed, so he prayed Isha. Then he came to him for Fajr when it was very bright, and said: "Stand and pray." So he prayed Fajr. Then he said: "What is between these two is the time."