Musnad Abu Ya'la al-Mawsili 108:172

Abu Khaythama narrated to us, Muhammad ibn Khazim narrated to us, Dawud ibn Abi Hind narrated to us, from Abu Nadra, from Jabir ibn Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) came out to his companions one night while they were waiting for the Isha [prayer]. He said: "The people have prayed and slept, while you are waiting for it. As for you, you are in a state of prayer as long as you are waiting for it." Then he said: "Were it not for the weakness of the weak and the old age of the elderly, I would have delayed this prayer until the middle of the night."