“Fasting three days of every month is like fasting for the entire year. The Ayyām al-Bīḍ (the white days) are the 13th, 14th, and 15th of the month.”

Narrated by al-Nasā’ī (No. 2420), Abū Ya‘lā (No. 7504), al-Ṭabarānī in al-Awsat (No. 7550), al-Ṣaghīr (No. 913), al-Kabīr (No. 2499), and al-Bayhaqī in al-Shu‘ab (No. 3570), from the ḥadīth of Jarrīr ibn ‘Abdullāh al-Bajālī, may Allah be pleased with him.
Authenticated in Ṣaḥīḥ al-Jāmi‘ (No. 3849) and in Ṣaḥīḥ al-Targhīb wa al-Tarhīb (No. 1040).

Brief Explanation of the Hadith

The Prophet, may Allah’s peace and blessings be upon him, encouraged his companions to seize opportunities for goodness. Among these is the consistent fasting of three days each month, due to the great reward it contains.
His statement, “Fasting three days of every month is like fasting for the entire year,” means that fasting three days in every month equals the reward of fasting the whole month, for every good deed is multiplied tenfold. Thus, one day counts as ten, and three days count as thirty — the equivalent of a full month. Whoever persists in this practice throughout his life attains the reward of fasting continuously. “The Ayyām al-Bīḍ are the mornings of the 13th...” That is, the white nights of the month are the 13th, ... of each lunar month, and they are so called because of the brightness of their nights due to the rising and fullness of the moon.
An exception is the 13th of Dhul-Ḥijjah, as it falls within the Days of Tashrīq, on which fasting is prohibited.
Several wisdoms have been mentioned for fasting these days, including the suitability of worship during the fullness of light in their nights, or gratitude to Allah for the complete illumination, and other explanations have also been proposed; all are possible reasons.
The ruling is that fasting three days of every month is a general Sunnah, whether on the Ayyām al-Bīḍ or other days. However, it is preferable that it be observed on the Ayyām al-Bīḍ.
Whoever is unable to fast on these days due to a valid excuse — such as travel, illness, menstruation, postpartum bleeding, or similar reasons — attains the reward by fasting three days in any part of the month.
The ḥadīth also demonstrates the vastness of Allah’s generosity and bounty, for He grants great reward for little. It further illustrates that a small, consistent act produces greater and more enduring benefit than a large, intermittent one.