Musannaf Ibn Abi Shayba 21:234

Yunus ibn Muhammad narrated to us, Layth ibn Sa'd narrated to us, from Yazid ibn Abi Habib, from Abu al-Khayr, from Abdullah ibn Amr, from Abu Bakr, that he said to the Messenger of Allah (may Allah bless him and grant him peace): "Teach me a supplication that I may supplicate with." He said: "Say: 'O Allah, I have wronged my soul with much wrongdoing, and none forgives sins except You, so forgive me with a forgiveness from You and have mercy upon me; indeed, You are the Forgiving, the Merciful.'"