It was narrated from Anas that someone asked the Messenger of Allah (ﷺ) about the time of Subh. The Messenger of Allah (ﷺ) commanded Bilal to call the Adhan when dawn broke. then the next day he delayed Fajr until it was very light, then he told him to call the Adhan and he prayed. Then he said:
"This is the time for the prayer."
English reference: Vol. 1, Book 7, Hadith 643
Arabic reference: Book 7, Hadith 649