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 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 is 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 of transmission is hasan, and Bayhaqi likewise graded it hasan. See the detailed sourcing given previously.On this subject there is also a report from Qatada bin Milhan Qaysi, that he was with the Prophet ﷺ and said, "The Prophet ﷺ used to command them to fast the White Days," and he would say, "That is fasting the whole lifetime." Abu Dawud (2449), Nasai (2430), Ibn Majah (1707), Ahmad (20316), and Ibn Hibban (3651) all narrated it from the hadith of Anas bin Sirin, from Ibn Milhan Qaysi, from his father, and he related it.As for Ibn Milhan, it is said he is Qatada bin Milhan Qaysi, and it is also said he is Milhan bin Shabal, father of Abdul-Malik bin Milhan, and it is also said that he is Minhal bin Milhan, father of Abdul-Malik. Ibn Main said, "That is a mistake." Munziri stated this.I say, this Ibn Milhan had no one narrate from him other than Anas bin Sirin, and no one has affirmed him as reliable, and for that reason he is counted among the unknown narrators. As for Ibn Hibban, he mentioned him in Thiqat, following his own practice of affirming unknown narrators as reliable, and he included his hadith in his Sahih. Hafiz said, "Acceptable," meaning when corroborated, and I have not found anyone who corroborated him.