Sahih Muslim 646 a
The Book of Mosques and Places of Prayer
It is recommended to pray Subh early, at the beginning of its time, when it is still dark; and the length of recitation therein
Muhammad b. 'Amr b. al-Hasan b. 'All reported: When Hajjaj came to Medina we asked Jabir b. Abdullah (about the timings of prayer as observed by the Prophet). He said: The Messenger of Allah (ﷺ) used to pray afternoon prayer in the midday heat; the afternoon prayer when the sun was bright; the evening prayer when the sun had completely set; and as for the night prayer, he sometimes delayed and sometimes (observed it) at earlier hours. When he found them (his Companions) assembled (at earlier hours) he (prayed) early. and when he saw them coming late, he delayed the (prayer). and the morning prayer the Messenger of Allah (ﷺ) observed in the darkness before dawn.