Musannaf Ibn Abi Shayba 3:854

Waki’ narrated to us, from Sufyan, from Habib ibn Abi Thabit, from Nafi’ ibn Jubayr, who said: Umar wrote to Abu Musa: "Pray Dhuhr when the sun has passed its zenith, and pray Asr while the sun is white and bright. Pray Maghrib when the night has mingled [with the day], and pray Isha at any time of the night you wish. And pray Fajr when the light has dawned."