Musnad Abu Ya'la al-Mawsili 1:32

Harun ibn Ma'ruf narrated to us, Abdullah ibn Wahb narrated to us, Amr ibn al-Harith informed me, from Yazid ibn Abi Habib, from Abu al-Khayr, that he heard Abdullah ibn Amr ibn al-As saying: Abu Bakr al-Siddiq said to the Messenger of Allah (may Allah bless him and grant him peace): "O Messenger of Allah, teach me a supplication that I may supplicate with in my prayer and in my house." 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.'"