Musannaf Ibn Abi Shayba 3:6428

Kathir ibn Hisham narrated to us, from Ja'far ibn Burqan, who said: Umar ibn Abd al-Aziz wrote to us: "Perform the Maghrib prayer when the fasting person breaks his fast." Then he mentioned to me that some people hasten the Isha prayer before the whiteness of the horizon from the west has departed. Do not pray it until the whiteness of the horizon from the west has departed and the darkness of the night has covered it. Whatever you hasten after the departure of the whiteness of the horizon from the west, it is better and more correct. Know that part of its perfection and hitting its proper time is what I have mentioned to you in this letter of mine regarding the departure of the whiteness of the horizon, for it is a remainder of the remnants of the day.