Maliks Muwatta Book 1, Hadith Number 3.
Section : The Times of Prayer.
Yahya related to me from Malik from Zayd ibn Aslam that Ata ibn Yasar said, “A man came to the Messenger of Allah, may Allah bless him and grant him peace, and asked him about the time of the subh prayer. The Messenger of Allah, may Allah bless him and grant him peace, did not answer him, but in the morning he prayed subh at first light. The following morning he prayed subh when it was much lighter, and then said, ‘Where is the man who was asking about the time of the prayer?’ The man replied, ‘Here I am, Messenger of Allah.’ He said, ‘The time is between these two.'”