It is narrated from Amr bin Dhi Murr and Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "O Ali! Shall I not teach you a supplication which, if you recite it, then even if you had sins as many as the specks of dust, they would be forgiven for you, although you are already forgiven? Say: 'Allah, la ilaha illa anta al-Hakim al-Karim, tabarakta, subhanaka Rabb al-'Arsh al-'Azim' (O Allah! There is no deity worthy of worship except You, the Wise, the Generous; Blessed are You, Glory be to You, Lord of the Mighty Throne)."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5041