Musnad Ahmad 32:1020

Muhammad ibn Abdullah narrated to us, Humayd narrated to us, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, was asked about the time of the morning prayer. So he prayed when the dawn broke, then he waited until it became bright, until it was bright. Then he said: "Where is the one who asked about the time of the morning prayer?" He said: "The time is between these two."