´It was narrated that Umm Salamah said:` "The Messenger of Allah used to fast three days of every month: the Monday and Thursday of the first week and the Monday of the following week."
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Explanation & Benefits
Hafiz Muhammad Ameen
(1) "He used to command." That is, as a recommendation (mustahabb).
(2) "The first Thursday." In previous narrations, mention is made of the first Monday. The intent is that if the first Thursday arrived, then he would fast on Thursday, Monday, and then the next Monday; and if Monday came first at the beginning of the month, then he would fast on Monday, Thursday, and then the following Thursday. That is, the three fasts would be confined to Mondays and Thursdays. Whether the beginning is from Thursday or Monday, there is no difference.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2421