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

"The Messenger of Allah (saw) used to delay the later Isha."* (Sahih) *It is described as the later Isha prayer because the Maghrib prayer is sometimes called Isha prayer, but it is the first Isha. Some scholars are of the opinion that it is disliked to call Maghrib Isha without qualifying it as the first Isha. See Fath Al-Bari.