Musannaf Ibn Abi Shayba 3:968

Ibn Numayr and Abu Usamah narrated to us, from Ubaydullah, from Sa'id ibn Abi Sa'id, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Were it not that I would burden my nation, I would have delayed the Isha prayer until a third of the night or half of the night."