Musannaf Ibn Abi Shayba 3:987

Muhammad ibn Ubayd narrated to us, from al-Awwam ibn Hawshab, who said: I said to Mujahid, "The *shafaq* (twilight)?" He said, "Do not say *al-shafaq*, for *al-shafaq* is from the sun. Rather, say *humrat al-ufuq* (the redness of the horizon)."