Sunan an-Nasa'i, Book 22, Hadith 324 (In-Book) ⓘ
It was narrated that Al-Hurr bin Sayyah said: "I heard Ibn 'Umar say" "The Prophet used to fast three days of each month.
أَخْبَرَنَا يُوسُفُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا حَجَّاجٌ، عَنْ شَرِيكٍ، عَنِ الْحُرِّ بْنِ صَيَّاحٍ، قَالَ سَمِعْتُ ابْنَ عُمَرَ، يَقُولُ كَانَ النَّبِيُّ صلى الله عليه وسلم يَصُومُ ثَلاَثَةَ أَيَّامٍ مِنْ كُلِّ شَهْرٍ .
also referenced under: Hadith 2413 (Reference) & Volume 3, Book 22, Hadith 2415 (DarusSalam)