What constitutes the Shafaq (twilight) that signals the end of Maghrib and the start of Isha' time?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
The prevailing opinion among Imams, including Ibn 'Umar, Ibn 'Abbas, Al-Shafi'i, and two companions of Abu Hanifa, is that the Shafaq which ends the time of Maghrib and begins the time of Isha' is the redness (Al-Humrah). This is supported by narrations that the Prophet (peace be upon him) prayed when the first twilight (redness) disappeared. If one is in a location where the horizon is obscured by walls, one should wait until the whiteness disappears to confirm the setting of the redness.
Supporting text
It is reported from Anas and Abu Hurairah that the Shafaq is the whiteness (Al-Biyad). This view is held by Al-Awza'i and Abu Hanifa. The evidence presented for this is the narration that the Prophet (peace be upon him) prayed Isha' when the moon disappeared on the third night, and when the horizon became completely dark.