Praise be to Allah.
Firstly:
It was narrated that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever fasts Ramadan out of faith and in hope of reward, his previous sins will be forgiven.” Narrated by al-Bukhaari (38) and Muslim (759).
Fasting Ramadan is only achieved by fasting all the days. If someone does not fast all of it, he cannot be described as having fasted Ramadan; rather he may be described as having fasted part of it, or having fasted the month except a few days.
Al-Kermaani (may Allah have mercy on him) said:
The words “fasted Ramadan” means in the month of Ramadaan. If you ask: Is it sufficient to do the least of what may be described as fasting, even if he fasted only one day, to be included in this hadith?
I say: Customarily it cannot be said that someone fasted Ramadan unless he fasted the entire month. The context clearly indicates that.
If you ask about the one who had an excuse, such as one who is sick, and did not fast during the month, and if he had not been sick he would have fasted, and his intention was that he would have fasted, were it not for that reason: is he included in this hadith?
I say: Yes, just as the sick person who prays sitting because he has an excuse will have the reward of one who prays standing. This was stated by the leading scholars.
End quote from al-Kawaakib ad-Daraari (1/159).
Shaykh Mahmoud Khattaab as-Subki (may Allah have mercy on him) said:
The words “The one who fasts Ramadan…” refer to one who fasts all the days of the month.
With regard to the one who breaks the fast on some days with no excuse, he will not attain this reward.
The one who breaks the fast due to a valid excuse will have the reward, if he does what he is obliged to do of making up the missed days or feeding the poor instead, just as the one who prays sitting because of an excuse will have the reward of one who prays standing.
End quote from al-Manhal al-‘Adhb al-Mawrood Sharh Sunan Abi Dawood (7/308).
Secondly:
Anyone in the same position as this person should take note that if he has missed out on the opportunity to attain this great virtue (of fasting the entire month of Ramadan), there are other opportunities of attaining virtue that he should hasten to seek, the most important of which is repenting sincerely.
Please see the answer to question no. 13693.
In Ramadan, there are other righteous deeds that will expiate sins apart from fasting. One of these deeds is spending the last ten nights of Ramadan in prayer, out of faith and in hope of reward. Perhaps the one who spends these nights in prayer will be enabled to find Laylat al-Qadr; spending that night in prayer will bring the same forgiveness of sins as fasting Ramadan.
It was narrated that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever spends Laylat al-Qadr in prayer out of faith and in the hope of reward, his previous sins will be forgiven.” Narrated by al-Bukhaari (35) and Muslim (760).
See question no. 25 for information on the most important opportunities for doing good deeds in Ramadan.
We also advise you to read the books al-Khisaal al-Mukaffirah li’dh-Dhunoob by al-Haafiz Ibn Hajar al-‘Asqallaani, and al-Khisaal al-Mukaffirah li’dh-Dhunoob by Shams ad-Deen ash-Sharbeeni.
And Allah knows best.
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