Musnad Ahmad 32:932

Yahya narrated to us, from Humayd, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, was asked about the time of the morning prayer. He prayed when the dawn broke. Then, the next day, he prayed after it had become bright. Then he said: "Where is the one who asked about the time of the morning prayer? It is between these two."