The Intention for Fasting on Tasua and Ashura with Evidences and Their Virtues
Muslims are encouraged to perform as many sunnah acts as possible, including the Tasua fast on the 9th of Muharram and Ashura on the 10th. In a hadith narrated by Bukhari and Muslim, Prophet Muhammad (peace be upon him) said, “Whoever fasts one day in the way of Allah (a voluntary fast), Allah will keep his face away from the Hellfire for seventy years.”
The intention for Tasua fasting: Nawaitu shauma ghadin ‘an adai sunnatit Tasua lillahi taala (I intend to fast the sunnah of Tasua tomorrow for the sake of Allah the Most High). The intention for Ashura fasting: Nawaitu shauma ghadin ‘an ada’i sunnatil Asyura lillahi ta’ala (I intend to fast the sunnah of Ashura tomorrow for the sake of Allah the Most High). The intention can be uttered during the day if you haven’t eaten, drunk, or done anything else that breaks the fast since Fajr.
Some of the evidences encouraging this fast include a hadith narrated by Muslim about fasting on the ninth day of Muharram, and a hadith narrated by Bukhari stating that the Prophet (peace be upon him) highly prioritized the Ashura fast after Ramadan.
Its virtues: the fast of Muharram is the best fast after Ramadan (narrated by Muslim), it's performed in a sacred month (al-asyhurul hurum), and the Ashura fast wipes away the sins of the previous year (narrated by Muslim). Imam Nawawi explains that this expiation is for minor sins, while major sins require sincere repentance.
https://mozaik.inilah.com/ibad