Musnad Ahmad 1:8

Hashim ibn al-Qasim narrated to us, he said: al-Layth narrated to us, he said: Yazid ibn Abi Habib narrated to me, from Abu al-Khayr, from Abdullah ibn Amr, 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 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.'" And Yunus said: "Great (kabiran) [instead of 'much' (kathiran)]." Hasan al-Ashyab narrated it to us, from Ibn Lahi'ah, he said: "Great (kabiran)."