What are the prescribed times for Isha prayer?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
The time for Isha prayer begins when the darkness deepens (Yaghsaq al-Layl) and the redness of the horizon disappears, lasting until the first third of the night has passed. Delaying Isha is recommended, especially since the night is long. Mu'adh was instructed not to hasten Isha but to delay it until the red twilight vanishes. Umar stated that whoever sleeps through this time (after the first third) should have God deny him sleep.
Supporting text
The Prophet used to delay Isha sometimes, and sometimes hasten it if people had gathered. Umar specified that the time extends until the disappearance of the red horizon into the first third of the night.