Narrated ‘Aishah (RA): The Prophet (SAW) delayed (the ‘Isha’ prayer) one night till a great part of the night passed, then he went out and offered the prayer, and said, “This is the proper time for it; were it not that I would impose a burden on my followers.” [Reported by Muslim].
Bulugh al-Maram, Book 2, Hadith 158