Mujahid (may Allah have mercy on him) said: I used to lead my master, Sayyiduna Qays bin al-Sa'ib (may Allah be pleased with him), (to show him the way), and he would ask: Has the sun passed its zenith? When I said: Yes, he would pray Zuhr and say: This is how the Messenger of Allah (peace and blessings be upon him) used to do. And the Prophet (peace and blessings be upon him) used to pray Asr while the sun was white and bright, and he used to pray Maghrib when the fasting person would be in doubt whether it was time to break the fast (that is, he prayed it very early), and he used to pray Fajr when the light had spread across the sky.
Hadith Referenceالمعجم الكبير للطبراني / باب القاف / 16022