Musannaf Ibn Abi Shayba 3:970

Ishaq bin Mansur narrated to us, from Muhammad bin Muslim, from Amr bin Dinar, from Ata, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) delayed the Isha prayer one night. He came out while his head was dripping [with water]. He said: "Were it not that I would make it difficult for my nation, I would have set the time for this prayer at this time."