Sunan an-Nasa'i #3104
Sahih (Authentic)
Fasting
كُلُّ عَمَلِ ابْنِ آدَمَ لَهُ إِلاَّ الصِّيَامَ فَإِنَّهُ لِي وَأَنَا أَجْزِي بِهِ
The Messenger of Allah (ﷺ) said: 'Every deed of the son of Adam is for him, except fasting — it is for Me and I shall reward for it.'
Chain of Narration (Isnad)
Narrated Abu Huraira