كتاب المواقيت
Read the English translation first, with the original Arabic directly underneath.
It was narrated that Ibn 'Abbas said:
"The Prophet (ﷺ) 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 (ﷺ) 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.'"
إِنَّهُ الْوَقْتُ لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي