Musnad Ahmad 27:1242

Abu Ahmad al-Zubayri narrated to us, Isra'il narrated to us, from Fudayl, from Mujahid, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) delayed the Isha prayer until the people had slept, those who pray at night had prayed, and those who were awake had awakened. Then he came out, and the prayer was called, and he said: "Were it not that it would be difficult for my Ummah, I would have delayed it until this time."