Sahih Al Bukhari 1: Chapter 2, Hadith 41

(41) (What is said regarding the superiority of) a person who embraces Islam sincerely.

قَالَ مَالِكٌ:‏‏‏‏ أَخْبَرَنِي زَيْدُ بْنُ أَسْلَمَ، ‏‏‏‏‏‏أَنَّ عَطَاءَ بْنَ يَسَارٍ أَخْبَرَهُ، ‏‏‏‏‏‏أَنَّ أَبَا سَعِيدٍ الْخُدْرِيَّ أَخْبَرَهُ، ‏‏‏‏‏‏أَنَّهُ سَمِعَ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ، ‏‏‏‏‏‏يَقُولُ:‏‏‏‏ "إِذَا أَسْلَمَ الْعَبْدُ فَحَسُنَ إِسْلَامُهُ يُكَفِّرُ اللَّهُ عَنْهُ كُلَّ سَيِّئَةٍ كَانَ زَلَفَهَا، ‏‏‏‏‏‏وَكَانَ بَعْدَ ذَلِكَ الْقِصَاصُ الْحَسَنَةُ بِعَشْرِ أَمْثَالِهَا إِلَى سَبْعِ مِائَةِ ضِعْفٍ وَالسَّيِّئَةُ بِمِثْلِهَا، ‏‏‏‏‏‏إِلَّا أَنْ يَتَجَاوَزَ اللَّهُ عَنْهَا"

SAHIH HADITH:

Narrated Abu Sa'id Al Khudri:Allah's Messenger (saws) said, If a person embraces Islam sincerely, then Allah shall forgive all his past sins, and after that starts the settlement of accounts, the reward of his good deeds will be ten times to seven hundred times for each good deed and one evil deed will be recorded as it is unless Allah forgives it.

REFERENCE:

Sahih Al Bukhari 1: Chapter 2, Hadith 41