The Meaning of the Hadith: (Fasting three days of the month is fasting the whole year)
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Q502: The questioner asks: What is the meaning in the hadith: Sunan an-Nasa'i 2399 ? Are the three days at any time of the month, or are they on specific days?
A502: Fasting three days of every month, all of this is a confirmed sunnah (recommended practice), as the Prophet (peace and blessings be upon him) advised Abu Hurayrah (may Allah be pleased with him) to do so. It is for a full month; because the month is thirty days, and a good deed is rewarded ten times over, so each day is for ten days. This is the reason for specifying or legislating the fasting of three days of every month, so it is as if he fasted the whole month, meaning he has the reward of one who fasted the whole month. It is not fixed at a specific time of the month; rather, if he wishes, he fasts them at the beginning of the month, if he wishes in the middle, and if he wishes at the end. It is also not necessary to fast them consecutively; he may fast them together or separately. The matter is broad in this regard.
Fasting a lifetime means fasting three days each month
صوم ثلاثة أيام من الشهر صوم الدهرA502: Fasting three days of every month, all of this is a confirmed sunnah (recommended practice), as the Prophet (peace and blessings be upon him) advised Abu Hurayrah (may Allah be pleased with him) to do so. It is for a full month; because the month is thirty days, and a good deed is rewarded ten times over, so each day is for ten days. This is the reason for specifying or legislating the fasting of three days of every month, so it is as if he fasted the whole month, meaning he has the reward of one who fasted the whole month. It is not fixed at a specific time of the month; rather, if he wishes, he fasts them at the beginning of the month, if he wishes in the middle, and if he wishes at the end. It is also not necessary to fast them consecutively; he may fast them together or separately. The matter is broad in this regard.
Source
www.alifta.gov.sa
The Arabic text is copied verbatim from the original source, without any edits.