1a] Allaah said, “Fasting is for Me and I shall reward for it?”

1a]
2 – Allaah said concerningfasting: "I shall reward him for it." So he
connected the reward for fasting to Himself, because the reward for
righteous deeds is mentioned by number, and a good deed will be
rewarded between ten and seven hundred fold, many times. But with
regard to fasting, Allaah connected the reward to Himself without
specifying any number. Allaah is the most generous of those who are
generous, and the gift reflects the generosity of the giver. So the
reward of the one who fasts will be very great, without reckoning.
Fasting is patience in obeying Allaah, patience in keeping away from
the things forbidden by Allaah, and patience in bearing the decree of
Allaah, hunger, thirst, physical and mental weakness. So it combines
all three types of patience,thus the fasting person deserves to be
counted asone of the patient, and Allaah says (interpretationof the
meaning):
"Only those who are patient shall receive their reward in full,
without reckoning"
[al-Zumar 39:10]

And Allaah knows best.