Musnad Abu Ya'la al-Mawsili 133:27

Ahmad ibn Hatim narrated to us, Mu'tamir ibn Sulayman narrated to us, he said: A man was narrated to me who is called Bayan, he said: I said to Anas, "Narrate to me the time of the Messenger of Allah, may Allah bless him and grant him peace, for the prayer." He said: "He used to pray the Dhuhr when the sun declined, and he used to pray the Asr between your first prayer and the Asr. He used to pray the Maghrib at the setting of the sun, and he used to pray the Isha at the setting of the twilight, and he used to pray the Ghadah (Fajr) at the dawn when the sight begins to open. Everything between that is a time," or he said, "a prayer."