Musnad Ahmad 32:780

Hajjaj narrated to us, Shu'bah narrated to me, from Abu Sadaqah, the freed slave of Anas—and Shu'bah praised him well—he said: I asked Anas about the prayer of the Messenger of Allah, peace and blessings of Allah be upon him. He said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to pray Dhuhr when the sun passed the meridian, and Asr between these two prayers of yours, and Maghrib when the sun set, and Isha when the twilight disappeared, and Subh when the dawn broke, until the eyesight could see clearly."