Musannaf Ibn Abi Shayba 3:853

Waki’ narrated to us, from Sufyan, from al-Mughirah ibn al-Nu’man, from Ali ibn ‘Amr, who said: A letter from ‘Umar came to us: "Perform the Fajr prayer while the stars are intertwined and bright. Perform the Dhuhr prayer when the sun has passed from the center of the sky. Perform the ‘Asr prayer while the sun is white and pure. Perform the Maghrib prayer when the sun sets." And he granted a concession regarding the ‘Isha prayer.