The Messenger of Allah ﷺ said to a man, "Fast three days of every month, the thirteenth, the fourteenth, and the fifteenth."
قال رسول الله ﷺ لرجل: وصم من كل شهر، ثلاث عشرة، وأربع عشرة، وخمس عشرة.
Ibn Hibban narrated it in his Sahih (1887), and Bayhaqi in Dalail al-Nubuwwah (6/294), both from the hadith of Yahya bin Abdul-Rahman Arhabi, who said, "Ubayda bin Aswad narrated to me," from Qasim bin Walid, from Sinan bin Harith bin Musrif, from Talha bin Musarrif, from Mujahid, from Ibn Umar, and he related a long hadith, this being a part of it. The second part is in the book on prayer, and the third part of it is in the book on Hajj, on the merit of the day of Arafah. Its chain is hasan, and Bayhaqi likewise graded it hasan. See the detailed sourcing given earlier.