Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

that his father said: "A man came to the Prophet (saw) asking him about the times of prayer, and he did not answer him. He told Bilal to say the Iqamah at dawn broke, then he told him to say the Iqamah for Zuhr when the sun had passed its zenith and a person would say: 'It is the middle of the day,' but he (the Prophet (saw)) knew better. Then he told him to say the Iqamah for Asr when the sun was still high. Then he told him to say the Iqamah for Maghrib when the sun had set. Then he told him to say the Iqamah for Isha when the twilight had disappeared. Then the next day he told him to day the Iqamah for Fajr, at a time such that when after he had finished one would say: 'The sun has risen.' Then he delayed Zuhr until it was nearly the time of Asr compared to the day before. Then he delayed Asr, to a time such that when he finished, one would say: 'The sun has turned red.' Then he delayed Maghrib until the twilight was about to disappear. Then he delayed 'Isha' until one-third of the night had passed. Then he said: 'The time (for prayer) is between these times.'" (Sahih)