Musannaf Ibn Abi Shayba 21:591

Yazid ibn Harun narrated to us, from Hammad ibn Salamah, from Hisham ibn ‘Amr, from ‘Abd al-Rahman ibn al-Harith ibn Hisham, from ‘Ali, that the Prophet (may Allah bless him and grant him peace) used to say at the end of his Witr: "O Allah, I seek refuge in Your pleasure from Your anger, and I seek refuge in Your pardon from Your punishment, and I seek refuge in You from You. I cannot enumerate Your praise; You are as You have praised Yourself."