It was narrated from Anas that someone asked the Messenger of Allah (S.A.W) about the time of Subh. The Messenger of Allah (S.A.W) 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."
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا يَزِيدُ، قَالَ حَدَّثَنَا حُمَيْدٌ، عَنْ أَنَسٍ، أَنَّ سَائِلاً، سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ وَقْتِ الصُّبْحِ فَأَمَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم بِلاَلاً فَأَذَّنَ حِينَ طَلَعَ الْفَجْرُ فَلَمَّا كَانَ مِنَ الْغَدِ أَخَّرَ الْفَجْرَ حَتَّى أَسْفَرَ ثُمَّ أَمَرَهُ فَأَقَامَ فَصَلَّى ثُمَّ قَالَ " هَذَا وَقْتُ الصَّلاَةِ " .
Nesai
The Book of the Adhan (The Call to Prayer) • Hadith 642