Musnad Abu Ya'la al-Mawsili 129:84

'Abd al-A'la narrated to us, Mu'tamir narrated to us, he said: I heard Humayd narrating from Anas, who said: A man came to the Prophet (may Allah bless him and grant him peace) and asked him about the time of the dawn prayer. When we reached the next morning, he ordered that the prayer be established as soon as the dawn broke. When the next day came, he delayed it until it became bright, then he gave the order and the prayer was established, and he led us in prayer. Then he said: "Where is the one who asked about the time of the prayer? The time is between these two."