The Messenger of Allah ﷺ commanded us to fast three days of each month, the white days, the thirteenth, the fourteenth, and the fifteenth.
أخبرنا محمد بن عبد العزيز قال أنبأنا الفضل بن موسى عن فطر عن يحيى بن سام عن موسى بن طلحة
أمرنا رسول الله ﷺ أن نصوم من الشهر ثلاثة أيام البيض ثلاث عشرة وأربع عشرة وخمس عشرة
[Its isnad is hasan] Tirmidhi narrated it in the Book of Fasting, chapter on what has been said about fasting three days from every month, hadith no. 761, through the hadith of Yahya bin Sam, with this wording, and he said, "Hasan." It is also found in Kubra, hadith no. 2730. Ibn Khuzayma graded it sahih, 302/3, 303, hadith no. 2128, as did Ibn Hibban, hadith nos. 944 and 943.