Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

"The Prophet (saw) delayed Isha one night until part of the night had passed. Then Umar, may Allah be pleased with him, got up and called out: 'The prayer, O Messenger of Allah! The women and children have gone to sleep.' Then the Messenger of Allah (saw) came out with water dripping from his head, saying: 'This is (the best) time (for Isha), were it not that this would be too difficult for my Ummah.'" (Sahih)