كُلُّ عَمَلِ ابْنِ آدَمَ يُضَاعَفُ الْحَسَنَةُ بِعَشْرِ أَمْثَالِهَا إِلَى سَبْعِمِائَةِ ضِعْفٍ، قَالَ اللَّهُ عَزَّ وَجَلَّ: إِلاَّ الصَّوْمَ فَإِنَّهُ لِي وَأَنَا أَجْزِي بِهِ، يَدَعُ شَهْوَتَهُ وَطَعَامَهُ مِنْ أَجْلِي
Abu Huraira reported: The Prophet (ﷺ) said, "Every deed of the son of Adam is multiplied — a good deed receiving tenfold to seven hundredfold reward — except fasting, for Allah the Almighty says: 'It is for Me and I shall reward it Myself, for he gives up his desire and his food for My sake.'"
Sahih Bukhari·1904
fasting