Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said, “When one of you perfects his Islam, then every good deed he does is written as ten good deeds up to seven hundred times as much, and every bad deed he does is written as one bad deed.”

Source: Sahih Bukhari 42, Sahih Muslim 129

Grade: Muttafaqun Alayhi (authenticity agreed upon) according to Al-Bukhari and Muslim

عَنْ أَبِي هُرَيْرَةَ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا أَحْسَنَ أَحَدُكُمْ إِسْلَامَهُ فَكُلُّ حَسَنَةٍ يَعْمَلُهَا تُكْتَبُ لَهُ بِعَشْرِ أَمْثَالِهَا إِلَى سَبْعِ مِائَةِ ضِعْفٍ وَكُلُّ سَيِّئَةٍ يَعْمَلُهَا تُكْتَبُ لَهُ بِمِثْلِهَا

42 صحيح البخاري كِتَاب الْإِيمَانِ بَاب حُسْنُ إِسْلَامِ الْمَرْءِ

129 صحيح مسلم كتاب الإيمان باب إذا هم العبد بحسنة كتبت وإذا هم بسيئة لم تكتب