The one deed Allah kept for Himself
Every other good deed has a stated multiplier.
إِلاَّ الصَّوْمَ فَإِنَّهُ لِي وَأَنَا أَجْزِي بِهِ
Illā-ṣ-ṣawma fa-innahu lī wa anā ajzī bih
Except fasting. It is Mine, and I will reward it Myself.
Where this comes from
Every deed of the son of Adam is multiplied, ten to seven hundred times over. Then the numbers stop. Fasting has no stated figure because nobody sees it. You could break it in a locked room and no one would know. That secrecy is the reason it was kept back.
The full narration
It was narrated from Abu Hurairah that the Messenger :of Allah (saas) said: "Every deed of the son of Adam will be multiplied for him, between ten and seven hundred times for each merit. Allah said: 'Except for fasting, for it is for Me and I shall reward for it.'" (sahih)
Source
Sunan Ibn Mājah · 3823
Ṣaḥīḥ
- Al-Albani: Sahih
- Muhammad Fouad Abd al-Baqi: Sahih
- Shuaib Al Arnaut: Sahih
- Zubair Ali Zai: Sahih