Based on Lata’if al-Ma’arif
By Ibn Rajab al-Hanbali 
Abu Hurayra, may God be pleased with him, relates that the Prophet, peace upon him, said: “The reward of every action undertaken by the human being is for him—and good deeds will be rewarded from ten up to seven hundred times—God says: ‘Except for fasting, that is for Me and I will determine its reward. He [the fasting person] has abandoned his lawful desire for sexual intimacy, food, and drink for my sake. The fasting person has two delights: The delight he experiences at the time he breaks his fast, and the delight he will have when he meets his Lord. The foul odor that sometimes emerges from the mouth of the fasting person is sweeter with God than the fragrance of musk.’ ” 
This prophetic tradition contains many valuable lessons. Some of these have been expounded on by Ibn Rajab al-Hanbali in his wonderful book, Lata’if al-Ma’arif fima li Mawasim al-‘Am min al-Wadha’if (The Intricacies of Knowledge Concerning What Each Season of the Year Contains of Religious Duties). We will endeavor to share some of those lessons based on his explanation of the tradition mentioned above. What follows is my translation of his discussion with only some very minor abridgements of the original text.
First of all, in the tradition mentioned above, fasting is exempted from the deeds whose reward is multiplied up to seven hundred times. We understand from this that its reward transcends this particular numerical value. Rather, God multiplies its reward many times over what is mentioned in the tradition, with no specific limitation.
One reason for this is that fasting is a manifestation of patience and God has said of the person displaying patience: Rather, the patience ones will be given their reward without numerical limitations (40:10) Consistent with this understanding, it has been related from the Prophet, peace upon him, that he called Ramadan the “Month of Patience.”  In another narration he said, peace upon him: “Fasting is half of all patience.” 
Patience has three manifestations: Patience in dealing with the difficulty or discomfort caused by obeying God; patience in dealing with the difficulty or discomfort caused by refraining from that which God has prohibited; and patience in dealing the pain of the calamities conveyed by the divine decree. All three types are found in fasting for it involves patience in obeying God, patience in refraining from what He has prohibited of desirable things, and patience in dealing with the pain of hunger and thirst that can afflict the fasting person.
The reward for patiently enduring the rigors and hardships of the fast is Paradise. Salman al-Farisi, may God be pleased with him, relates from the Prophet, peace upon him, in a tradition dealing with the virtue of Ramadan: “It is the month of patience, and the reward of patience is Paradise.”  Al-Tabarani relates a similar tradition from Ibn ‘Umar, may God be pleased with him and his father. He says: “Fasting is for God. Hence, no one knows its true reward except Him.” This tradition is attributed both to the Prophet, peace upon him, and to Ibn ‘Umar, may God be pleased with him and his father. The latter attribution is more accurate.
It should be known that multiplication of the reward for our deeds can be for a variety of reasons. Among them is the nobility of the place they occurred in such as the Sacred Mosque. [For example] the prayer is multiplied in the two mosques of Mecca and Medina. This has been affirmed in a tradition mentioned by Imam al-Bukhari. The Prophet, peace upon him, said: “Prayer in this mosque of mine is equivalent to a thousand prayers in any other mosque.” 
A similar multiplication of reward occurs for a fast that is undertaken in the Sacred Mosque. Imam Ibn Majah, with a weak chain of transmission, relates from Ibn ‘Abbas, may God be pleased with him and his father, that the Prophet, peace upon him, said: “Whosoever attains to Ramadan while in Mecca, fasting its days and spending part of its nights in prayer, God has ordained a reward equivalent to the worship of a thousand Ramadans fasted in any other place.” 
Another reason for the multiplication of the reward of religious deeds is the nobility of the time [those deeds are performed in], such as Ramadan and the first ten days of Dhu’l-Hijja. In the tradition related by Salman, which we have previously mentioned, concerning the virtue of the month of Ramadan, [the Prophet, peace upon him, said]: “Anyone performing a voluntary good deed during it, it is like one performing an obligatory good deed at another time, and anyone who does an obligatory act during it, it is as if he has done seventy obligatory acts at another time.” Anas, may God be pleased with him, mentioned: “The Prophet, peace upon him, was asked, ‘Which type of charity is best?’ He responded, ‘Charity done during Ramadan.’ ” 
It is also related that the Prophet, peace upon him, said: “A lesser pilgrimage (‘Umrah) performed in Ramadan is equivalent to a greater pilgrimage (Hajj).” Or he said: “A greater pilgrimage performed in my company.” 
Abu Bakr b. Abi Maryam mentioned that his teachers used to say: “When Ramadan comes try to be excessively charitable, because spending during Ramadan is like spending in the Way of God, and proclaiming the Glory of God (Subhanallah) one time [during Ramadan] is more virtuous than a similar proclamation made a thousand times outside of Ramadan.” Ibrahim al-Nakha’i said: “Fasting a day in Ramadan is better than fasting a thousand other days, and proclaiming the Glory of God once in Ramadan is better than glorifying Him a thousand times during other days, and a single unit of prayer during Ramadan is better than a thousand units of prayer performed during other days.”
Just as the reward of fasting is multiplied more than the reward of other acts, the reward of the fast of Ramadan is multiplied more than that of any other fast. This is because of the nobility of its time and because it is the fast that God has made obligatory for His servants. He has made its fasting one of the pillars of Islam upon which the religion is based.
The reward for the fast might be multiplied for other reasons, such as the nobility of the fasting person and his/her nearness to God, or the level of their piety, just as the reward for this [Muslim] community has been multiplied over the reward the previous communities received for their righteous deeds.
Another version of the tradition we mentioned initially states: “Every action the human being does is a source of expiation for his [misdeeds], except fasting, it is for Me and I will reward him for it.”  Among the nicest explanations of this is that given by Sufyan b.’Uyayana, he said: “This is one of the nicest and most insightful traditions [concerning the fundamental teachings of our religion]. When the Day of Judgment comes God will take account of His servants and all those he [the servant] has oppressed will be recompensed from the reward of his good deeds until only the reward of fasting remains. At that point God will recompense the remaining oppressed parties [from His bounty] and enter the servant into Paradise on the reward of his fast.” This narration has been recorded by Imam al-Bayhaqi , and others.
Based on this meaning we understand that the fast is for God in that no one can take the reward of the fast from the one who has undertaken it. Rather his reward is stored up with God.
In a related issue, it has been said that the reward of all of the religious actions might be a source of atonement for sins [that do not include acts of oppression] until none of that reward remains. It has been related that on the Day of Judgment [a person’s] good deeds will be weighed against [his] bad deeds with the good deeds atoning for the bad ones. If after this process any good remains on that basis its subject doer will enter Paradise. This opinion has been expressed by Sa’id b. Jubayr and others. It also has been related from the Prophet, peace upon him, by Imam Hakim from the narrations of Ibn ‘Abbas, may God be pleased with him and his father.
Based on what has been mentioned [concerning acts of oppression] it is possible to say that the reward of fasting is not removed by this process of atonement for one’s sins, or by anything else. Rather, its reward is held back and then given in full to the servant and based on that he will enter Paradise.
 Ibn Rajab al-Hanbali, Lata’if al-Ma’arif fima li Mawasim al-‘Am min al-Wadha’if (Damascus: Dar Ibn Kathir, 1996/1416) 283-287.
 Bukhari, #1894, and Muslim #2701
 Abu Dawud, #2428, and Ibn Majah, #1741
 Tirmidhi, #3519
 Ibn Khuzama, #1887
 Bukhari, #1190
 Ibn Majah, #3117
 Tirmidhi, #663
 Bukhari, #1863, and Muslim, #3028
 Ahmad, 2/257 and 2/273 In the extant versions of Imam Ahmad’s Musnad, the word “Kaffara” (atonement)” is not mentioned. Fasting being a source of atonement (Kaffara) is mentioned by Imam al-Bukhari, #1895.
 See Shu’ab al-Iman, 3:295 #3582