No ostentation in fasting
…"except fasting. It is (exclusively) for Me and I (alone) will reward it." This is because no one can know that the person is fasting except Allah The Almighty, as fasting is fulfilled through intention and abandonment, unlike other acts of worship that are done by performing certain acts and movements. This is why fasting cannot be apparently spoiled by ostentation except in a few cases. Allah The Almighty says: "for Me." There is a Dha‘eef Hadeeth that reads: "There is no ostentation in fasting." [Al-Bayhaqi] If this Hadeeth was Saheeh, it would settle the issue.
The greatest of secrets
The secrecy of fasting is greater than all other acts of worship as Allah The Almighty is the One who knows that which deceives the eyes and what the breasts conceal, as well as the slips of the tongue and any other thing that might affect one's fasting and He rewards the slave accordingly. Thus, fasting is one of the greatest acts of worship where Ihsaan (excellence of worship) is fulfilled! The Prophet, , said: "Ihsaan is to worship Allah as if you see Him. And even though you do not see Him, He sees you!" [Muslim and others]
Nothing can possibly be deducted thereof to requite injustice
This might mean that on the Day of Judgment, the wronged people would take from the reward of the good deeds of the son of Aadam except the reward of fasting according to the opinion of some scholars. The Prophet, , said: "Every (sinful) deed can be expiated; and fasting is for Me, and I will give the reward for it." [Al-Bukhari] This narration can be interpreted by that of Ahmad which reads: "All the deeds of the son of Aadam are expiation (for his sins) except fasting as it is (exclusively done) for Me and I (alone) will reward it." [Ahmad]. This might be considered a weak opinion as it is refuted by what was narrated in Saheeh Muslim: "They (the wronged people) will even clutch at fasting."
Every act of worship or act of obedience has a certain reward, as the good deed is multiplied a tenfold up to seven hundred fold reward, but the reward of fasting is only known to the One who has the depositories of heavens and earth, as it is different in reward from all other acts of worship. The reward of fasting is not multiplied like the reward of all other acts of worship, rather it is scooped in handfuls bountifully and generously and cannot be counted except by Allah The Almighty. Saying: "Fasting is for Me, and I will give the reward for it,"indicates the greatness of the reward of fasting as it is similar to the One who grants it. Abu Hurayrah, may Allah be pleased with him, narrated that The Prophet, , said: "Every (good) deed of the son of Aadam would be multiplied, a good deed receiving a tenfold to seven hundred fold reward. Allah The Almighty said, 'With the exception of fasting, for it is done for Me and I will give reward for it.’" [Muslim] This means that Allah The Almighty will grant people a great reward for fasting without specifying the amount of this great reward.
Patience
Fasting includes all kinds of patience as it entails being patient with acts of obedience, with prohibitions and disturbances. Allah The Almighty Says (what means): {Indeed, the patient will be given their reward without account.} [Quran 39:10].
Why fasting is exclusively for Allah The Almighty
Saying: "With the exception of fasting, for it is done for Me and I will give a reward for it," was interpreted with what was said in the narration of Muslim: "…for one abandons his desire, food and drink for My sake." [Al-Bukhari and Muslim] This is the wording of Muslim. Sincerity in fasting is greater than in any other act of worship. Fasting is considered a secret whose reality can be known only to Allah The Almighty – not only with regard to the intention, as this is something common in all other acts of worship, but with regard to the reality of performing the act of worship itself. For instance, the one who offers the prayer in a sound manner would have performed the prayer outwardly, but only Allah The Almighty knows the secrets of his inner self. However, fasting is different as its hidden reality is almost similar to its apparent one and the only One who can know its reality is Allah The Almighty.