Musannaf Ibn Abi Shayba 3:4566

Abu Bakr narrated to us, he said: Yazid bin Harun narrated to us, from Hammad bin Salamah, from Hisham, from Abd al-Rahman bin al-Harith bin Hisham, from Ali, that the Prophet, peace and blessings of Allah be upon him, used to say at the end of his Witr: "O Allah, I seek refuge in You from You. I cannot enumerate Your praise; You are as You have praised Yourself."