Sunan Ibn Majah Book 34, Hadith 8
"The Messenger of Allah (saas) often used to say: 'Allahumma thabbit qalbi 'ala dinika .' A man said: 'O Messenger of Allah! Do you fear for us when we have believed in you and in (the Message) that you have brought?' He said: 'Hearts are between two of the fingers of the Most Merciful, and He controls them.'" (Hasan)Al-A'mash (one of the narrators) indicated with his fingers.
"The Messenger of Allah (saas) used to say: 'Allahummanfa'ni bima 'allamtani, wa 'allimni ma yanfa'uni, wa zidni 'ilman, wal-hamdu lillahi 'ala kulli hal, wa a'udhu billahi min 'adhabin-nar (O Allah, benefit me by that which You have taught me, and teach me that which will benefit me, and increase me in knowledge. Praise is to Allah in all situations, and I seek refuge with Allah from the torment of the Fire).'"
he said to the Messenger of Allah (saas): "Teach me a supplication which I can say during my prayer." He said: "Say: Allahumma inni zalamtu nafsi zulman kathiran wa la yaghfirudh-dhunub illa Anta, faghfirli maghfiratan min 'indika warhamni, innaka Antal-Ghafurur-Rahim (O Allah, I have wronged myself greatly and no one forgives sins but You, so grant me forgiveness from You and have mercy on me, for You are the Oft-Forgiving, Most Merciful)."