Musannaf Ibn Abi Shaybah

Hazrat Nafi’ bin Jubair narrates that Hazrat Umar wrote a letter to Hazrat Abu Musa in which it was written: Perform the Zuhr prayer when the sun declines; perform the Asr prayer when the sun is white and bright; perform the Maghrib prayer when night and day merge with each other; perform the Isha prayer at night whenever you wish; and perform the Fajr prayer when the light spreads.