Best Dua(supplication) for forgiveness of sins
Jabir ibn Abdullah reported: A man came to the Prophet, peace and blessings be upon him, and he said, “O my sins! O my sins!”
The Prophet said to him, قُلِ
اللَّهُمَّ مَغْفِرَتُكَ أَوْسَعُ مِنْ ذُنُوبِي وَرَحْمَتُكَ أَرْجَى عِنْدِي مِنْ عَمَلِي
“Say: O Allaah, your forgiveness is vaster than my sins, and your mercy is more hopeful to me than my deeds.
The man said it, then the Prophet said, “Repeat it.” The man repeated it, then the Prophet said, “Repeat it again.” The man repeated it, then the Prophet said, “Stand up, for Allaah has forgiven you.”
[Hakim #1994; Shu’ab al-Imān 6622-Sahih (authentic) according to Al-Suyuti]