Sunan an-Nasa'i, Hadith 2365 ⓘ
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."
أَخْبَرَنِي أَبُو بَكْرِ بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا أَبُو نَصْرٍ التَّمَّارُ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ عَاصِمٍ، عَنْ سَوَاءٍ، عَنْ أُمِّ سَلَمَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُ مِنْ كُلِّ شَهْرٍ ثَلاَثَةَ أَيَّامٍ الاِثْنَيْنِ وَالْخَمِيسَ مِنْ هَذِهِ الْجُمُعَةِ وَالاِثْنَيْنِ مِنَ الْمُقْبِلَةِ .
also referenced under: Book 22, Hadith 276 (In-Book) & Volume 3, Book 22, Hadith 2367 (DarusSalam)