´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."
Hadith Referenceسنن نسائي / كتاب الصيام / 2367
Hadith Gradingالألبانی:حسن لكن الأصح بلفظ وخميس | زبیر علی زئی:صحيح