Do You Have to Fast the Ayyamul Bidh Consecutively? Here's the Evidence and Explanation
The Ayyamul Bidh fast is a sunnah practice done on the 13th, 14th, and 15th of each Hijri month, except the 13th of Dhul-Hijjah. The recommendation comes from the hadith of the Prophet Muhammad (peace be upon him). From Abu Hurairah, he advised fasting three days each month, praying duha, and sleeping after witr (narrated by Al-Bukhari). The Prophet also recommended those specific dates (narrated by Tirmidhi).
As for the order, the Ayyamul Bidh fast doesn't have to be done consecutively. This is based on a narration by Aisha (may Allah be pleased with her) that the Prophet wasn't strict about a particular day (narrated by Muslim and At-Tirmidhi). Scholars give six timing options, like doing it back-to-back on the 13th-15th, on Mondays and Thursdays, or any three days in a month.
Its virtue is that this fast equals fasting for a whole year. The Prophet said, 'Fasting three days every month is like fasting for a lifetime' (narrated by Bukhari). This practice can also erase sins and increase rewards.
https://mozaik.inilah.com/ibad