Musannaf Ibn Abi Shayba 35:387

Abu Usama narrated to us, from Abdullah, from Nafi’, from Safiyya bint Abi Ubayd, that Umar ibn al-Khattab wrote to the commanders of the armies, setting a time for them for the prayer. He said: "Perform the Isha prayer when the twilight has disappeared. If you are occupied, then [the time is] between you and the passing of one-third of the night. Do not be distracted from the prayer. Whoever sleeps after that, may Allah not let his eye sleep," he said it three times.