مسند أحمد

It was narrated from Abu Bakr as-Siddeeq that he said to the Messenger of Allah (ﷺ): Teach me a du'a` that I may say in my prayer. He said: `Say: O Allah. I have wronged myself greatly and no one forgives sins except You, so grant me forgiveness from You and have mercy on me, for You are the Oft-Forgiving, Most Merciful.`