The Prophet ﷺ said, "(Allah said), 'Every good deed of Adam's son is for him except fasting; it is for Me. and I shall reward (the fasting person) for it.' Verily, the smell of the mouth of a fasting person is better to Allah than the smell of musk."  

عَنِ النَّبِيِّ ﷺ قَالَ كُلُّ عَمَلِ ابْنِ آدَمَ لَهُ إِلاَّ الصَّوْمَ فَإِنَّهُ لِي وَأَنَا أَجْزِي بِهِ وَلَخَلُوفُ فَمِ الصَّائِمِ أَطْيَبُ عِنْدَ اللَّهِ مِنْ رِيحِ الْمِسْكِ