Hazrat Safiyyah bint Abi Ubaid narrates that Umar bin Khattab wrote about the times of prayer in a letter to the commanders of the armies. He said: Perform the Isha prayer when the twilight disappears. But if you are occupied with something, then (the time) is between that and one-third of the night, and do not make yourselves appear busy in regard to the right of prayer. Whoever sleeps after this, then may Allah not grant sleep to his eyes. The Prophet (peace and blessings be upon him) said this three times.