It was narrated that Abu Sa'eed Al-Khudri said:"The Messenger of Allah [SAW] said: 'If a person accepts Islam, such that his Islam is good, Allah will decree reward for every good deed that he did before, and every bad deed that he did before will be erased. Then after that will come the reckoning; each good deed will be rewarded ten times up to seven hundred times. And each bad deed will be recorded as it is, unless Allah, the Mighty and Sublime, forgives it.'"
أَخْبَرَنِي أَحْمَدُ بْنُ الْمُعَلَّى بْنِ يَزِيدَ، قَالَ حَدَّثَنَا صَفْوَانُ بْنُ صَالِحٍ، قَالَ حَدَّثَنَا الْوَلِيدُ، قَالَ حَدَّثَنَا مَالِكٌ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ عَطَاءِ بْنِ يَسَارٍ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا أَسْلَمَ الْعَبْدُ فَحَسُنَ إِسْلاَمُهُ كَتَبَ اللَّهُ لَهُ كُلَّ حَسَنَةٍ كَانَ أَزْلَفَهَا وَمُحِيَتْ عَنْهُ كُلُّ سَيِّئَةٍ كَانَ أَزْلَفَهَا ثُمَّ كَانَ بَعْدَ ذَلِكَ الْقِصَاصُ الْحَسَنَةُ بِعَشْرَةِ أَمْثَالِهَا إِلَى سَبْعِمِائَةِ ضِعْفٍ وَالسَّيِّئَةُ بِمِثْلِهَا إِلاَّ أَنْ يَتَجَاوَزَ اللَّهُ عَزَّ وَجَلَّ عَنْهَا " .
Nesai
The Book Of Faith and its Signs • Hadith 4998