I heard the Messenger of Allah ﷺ say, "Fasting the month of patience and fasting three days of every month is equivalent to fasting the entire year."
حدثنا أبو كامل حدثنا حماد عن ثابت البناني عن أبي عثمان النهدي أن
سمعت رسول الله ﷺ يقول صوم شهر الصبر وصوم ثلاثة أيام من كل شهر صوم الدهر
Its isnad is sahih. Abu Uthman Nahdi is Abdul-Rahman bin Mill, a trustworthy senior taabi who was mukhadram, having witnessed the Jahiliyyah and embraced Islam during the time of the Messenger of Allah ﷺ without meeting him. His trustworthiness was established earlier at 1410, along with the fact that he died in the year 100. We add here that Bukhari gave his biography in al-Saghir at 113, mentioning that he lived about 130 years. Ibn Saad also gave his biography in al-Tabaqat 7/1/69–70, as did Ibn Abu Hatim 2/2/283–284, and the Hafiz in al-Isabah, among the mukhadramun, 5:99–100. His father's name was Mill, with the mim vocalized as fatha, though it may also take damma or kasra, with the lam doubled. Nahdi is a relation to Nahd, with the nun vocalized as fatha and the ha as sakin, a great tribe from Quda'ah.The hadith was narrated by Nasai 1:327 through the route of Abdul-Ala from Hammad, with this same isnad, using the wording "the month of patience," omitting the word "fasting" from the beginning. Ibn Athir mentioned it in Jami al-Usul at 4483, attributing it only to Nasai. It will come again at greater length, with two isnads, also through the route of Hammad bin Salama, at 8974 and 10673. This longer version was narrated by Bayhaqi in Sunan al-Kubra 4:293, through the route of Hammad. See also what came earlier in the Musnad of Abdullah bin Umar at 6766.Regarding "the month of patience," Ibn Athir said, this is the month of Ramadan. The root meaning of sabr (patience) is restraint, and fasting was called sabr because it involves restraining oneself from food, drink, and intercourse.As for "three days," this is what is established in versions K and M. In version H it reads "for three days," and in version K it reads "and the fasting of three days," with the addition of the word "fasting," which we have added here based on it.