Musnad Ahmad 1:27

Hajjaj narrated to us, he said: Layth narrated to us, he said: Yazid ibn Abi Habib narrated to me, from Abu al-Khayr, from Abdullah ibn Amr ibn al-Aas, from Abu Bakr al-Siddiq: That he said to the Messenger of Allah, peace and blessings of Allah be upon him: "Teach me a supplication that I may supplicate with in my prayer." He said: "Say: 'O Allah, I have wronged my soul with much wrong, 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.'"