Musnad Ahmad 32:179

Isma'il narrated to us, Humayd al-Tawil informed us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) was asked about the time of the morning prayer. He said: So he commanded Bilal when the dawn broke, and he performed the iqamah for the prayer. Then, the next day, he waited until it was bright, until it was bright. Then he said: "Where is the one who asked about the time of the morning prayer? The time is between these two."