What are the specific conditions regarding twilight (Shafaq) for combining prayers due to rain?
Chapter on the Traveler's Prayer
Al-Mughni
Book of Prayer
Primary text
Combining prayers due to rain is permissible when darkness sets in, provided it is before the red twilight (Shafaq) has completely disappeared, as established by the practice of Ibn Umar. Narrations indicate that the governors would delay Maghrib and hasten Isha before the twilight vanished, and Ibn Umar prayed with them without objection. Al-Qasim and Salim also followed this practice during rainy nights.
Supporting text
The established Sunnah for combining prayers due to rain is to combine before the twilight disappears, whereas for travel, the combination is delayed until after the twilight has disappeared.