The exchange rate, stated in both directions
One side is multiplied and the other is not.
The words
إِذَا أَحْسَنَ أَحَدُكُمْ إِسْلاَمَهُ، فَكُلُّ حَسَنَةٍ يَعْمَلُهَا تُكْتَبُ لَهُ بِعَشْرِ أَمْثَالِهَا إِلَى سَبْعِمِائَةِ ضِعْفٍ، وَكُلُّ سَيِّئَةٍ يَعْمَلُهَا تُكْتَبُ لَهُ بِمِثْلِهَا
Idhā aḥsana aḥadukum islāmah, fa-kullu ḥasanatin yaʿmaluhā tuktabu lahu bi-ʿashri amthālihā ilā sabʿimiʾati ḍiʿf, wa kullu sayyiʾatin yaʿmaluhā tuktabu lahu bi-mithlihā
When one of you makes his Islam good, every good deed he does is written for him ten times over, up to seven hundred times. And every bad deed he does is written as one.
Where this comes from
The multiplier only runs one way. The other side is recorded at cost.
The full narration
Narrated Abu Huraira: Allah's Messenger (ﷺ) said, "If any one of you improves (follows strictly) his Islamic religion then his good deeds will be rewarded ten times to seven hundred times for each good deed and a bad deed will be recorded as it is
Source
Ṣaḥīḥ al-Bukhārī · 42
- Recorded in Ṣaḥīḥ al-Bukhārī, whose narrations are accepted as authentic