Narrated Ibn `Abbas: I never saw the Prophet seeking to fast on a day more (preferable to him) than this day, the day of 'Ashura', or this month, i.e. the month of Ramadan.
Related hadith on this topic
Explanation & Benefits
Hafiz Muhammad Ameen
Urdu marginal note:
There is no doubt whatsoever regarding the virtue of the blessed month of Ramadan; after it, the Day of Ashura, that is, the tenth of Muharram al-Haram, is the most virtuous. Many significant events took place on this day.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2372
Shaykh Muhammad Ibrahim bin Basheer
Benefit:
From this hadith, it is understood that the fasts of Ramadan and Ashura (the ninth day of Muharram) possess great virtue. Apart from these, all other days are equal in terms of fasting. If someone wishes, they may observe the fast of Dawud (صوم داؤدی) — that is, fasting one day and breaking the fast the next — or they may keep the fasts of Ayyam al-Beedh (ایام بیض), which are the fasts on the 13th, 14th, and 15th of each lunar month.
Source: Musnad al-Humaydi: Commentary by Muhammad Ibrahim bin Bashir, Page: 490