Musannaf Ibn Abi Shayba 3:848

Abu Khalid narrated to us, from Humayd, from Anas, that the Prophet, peace and blessings of Allah be upon him, was asked about the dawn prayer. He ordered Bilal, and he gave the call to prayer when the dawn appeared. Then, on the following day, he gave the call when it had become bright. Then he said, "Where is the questioner? The time is between these two."