The Messenger of Allah ﷺ commanded us to fast three days each month, the white days, the thirteenth, the fourteenth, and the fifteenth.
أمرنا رسول الله ﷺ أن نصوم من الشهر ثلاثة أيام البيض: ثلاث عشرة، وأربع عشرة، وخمس عشرة.
Tirmidhi (761), Nasai (2426) - the wording is his -, Ibn Khuzayma (2128), and Ibn Hibban (3566) all narrated it through the route of Yahya bin Sam, from Musa bin Talha, who said, "I heard Abu Dharr saying, while at Rabaza ( then he mentioned it)." Its chain is hasan because of Yahya bin Sam, who is al-Dabbi. A number of narrators transmitted from him, and Ibn Hibban mentioned him among the trustworthy narrators, so his hadith is deemed hasan if it has a basis. Tirmidhi also graded it hasan. Musa bin Talha is the son of Ubaydullah al-Taymi, a trustworthy and eminent narrator, and it is said that he was born during the time of the Prophet ﷺ.