Four things linked to ‘Āshūrā:
1. It is Sunnah to fast the month of Muharram and fasting of this month is the best, second to that of Ramadān. This is based on the Prophet’s ﷺ saying:
❝After the prescribed prayers the most excellent is the prayer offered in the middle of the night and the most excellent fast after (fasting) in the month of Ramadān is the fast in God’s month: al-Muharram.❞ Narrated by Muslim via Abū Hurayrah (r).
So, “unconditional voluntary fasting” is the best during the month of Muharram just as “unconditional voluntary prayers” are the best when done in as Qiyām al-Layl.
2. The best day of Muharram is ‘Āshūrā, which is the 10th of the month, and then it’s Tāsū’ā which is on the 9th.
Fasting on the day of ‘Āshūrā is an expiation for the sins of the past year as it has been authentically reported from the leader of mankind ﷺ.
It is Sunnah to combine between Tāsū’ā (9th) and ‘Āshūrā (10th) based on report of Ibn ‘Abbās (r) that the Messenger of Allāh ﷺ said, ❝If I live till the next (year), I would definitely observe fast on the 9th.❞ Imām Ahmad (r) deemed for this Hadīth to be sound for Ihtijāj (i.e. saw it sound for deriving rulings from it).
It isn’t Makrūh to only fast on the 10th.
3. Imām Ahmad (r) said:
❝If the start of the month of Muharram isn’t apparent to someone then he should fast 3 days.❞
One should do so as a precautinary act to attain certainty that they have attained the virtue (of fasting those two days), so, they fast the 9th, 10th, and 11th.
This is beneficial for those who ask about this due to difference between different countries with regards to the start of Muharram.
4. Our Fuqahā say that in this month the people should generously spend on his dependents (family).
Ibn Mansūr (r) asked Imām Ahmad (r),
❝Do you know of a Hadīth that says, ‘One who generously spends on his family on the day of ‘Āshūrā, Allāh will be generous to him for the entire year.’❞
He replied, ❝Yes. It was narrated by Sufyān b. ‘Uyaynah from Ja’far al-Ahmar from Ibrāhīm b. Muhammad b. al-Muntashir – who was one of the best people of his time – that the following had reached him: ‘One who generously spends on his family on the day of ‘Āshūrā, Allāh will be generous to him for the entire year.’❞
Sufyān b. ‘Uyaynah (r) said, ❝I have practised this [i.e. spending on the family] for fifty or sixty years and have found nothing but good in it.❞
It also said that Jābir (r) said, ❝We have practised this and found it to be so.”
Abū al-Zubair (r) and Shu’bah (r) have also said the same.
Yahyā b. Sa’īd (r) said: ❝We practised it and found it to be true.❞
I say: I have indeed practised this for years and found goodness. Some of our scholars who practised this have also told us the same thing.
— Sh. Muhammad ‘Abdul Wāhid al-Azharī (h)