The Messenger of Allah ﷺ used to command the observance of these three white days, saying, "They are equivalent to fasting the entire month."
أخبرنا محمد بن عبد الأعلى قال حدثنا خالد عن شعبة قال أنبأنا أنس بن سيرين عن رجل يقال له
أن رسول الله ﷺ كان يأمر بهذه الأيام الثلاث البيض ويقول هن صيام الشهر
[Its chain is daif] Ibn Majah narrated it in the Book of Fasting, chapter on what has been said about fasting three days each month, h: 1707, from the hadith of Shubah through this route, and it is in Kubra, h: 2737, and Ibn Hibban graded it sahih, h: 946, and Abu Dawud narrated it, h: 2449, through another route from Abdul-Malik through this route, and no one except Ibn Hibban considered him reliable.