Musnad Ahmad 32:279

Yazid ibn Harun narrated to us, Humayd informed us, from Anas, that a man asked the Prophet, peace and blessings of Allah be upon him, about the time of the morning prayer. So he ordered Bilal, and he gave the adhan when the dawn broke. Then he performed the iqamah and prayed. When it was the next day, he delayed until it became bright. Then he ordered him to perform the iqamah, and he prayed. Then he called the man and said: "Between this and this is the time."