The Prophet ﷺ said regarding fasting three days of each month, "It is like fasting the entire year along with its days of breaking the fast."
حدثنا وهب حدثنا شعبة
عن النبي ﷺ قال في صيام ثلاثة أيام من الشهر صوم الدهر وإفطاره
Its isnad is sahih, mentioned previously at 20243.