Musnad Ahmad 27:378

Aswad narrated to us, Israel informed 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 one praying had prayed, the one who was awake had woken up, the sleepers had slept, and those who perform Tahajjud had performed Tahajjud. Then he came out and said: "Were it not that it would be difficult for my Ummah, I would have commanded them to pray at this time, or this prayer, or something similar to that."