Musannaf Ibn Abi Shaybah

Hazrat Ali (may Allah be pleased with him) bin Amr says that a letter from Hazrat Umar came to us in which it was written: Offer the Fajr prayer at a time when the stars are bright and visible. Offer the Zuhr prayer when the sun has declined from the middle of the sky. Offer the Asr prayer when the sun is white and shining. Offer the Maghrib prayer when the sun sets, and he granted concession in the Isha prayer.