We give thanks to Almighty Allaah the Lord of the universe, and peace and blessings be upon our Prophet Muhammad, the Seal of the Prophets and Chief of the Messengers, and upon all his family and companions.
When the Prophet (ﷺ ) came to Medina, he found (the Jews) fasting on the day of ‘Ashura’ (i.e. 10th of Muharram).
They used to say:
“This is a great day on which Allah saved Moses and drowned the folk of Pharaoh. Moses observed the fast on this day, as a sign of gratitude to Allah.”
The Prophet (ﷺ ) said,
ﺃَﻧَﺎ ﺃَﻭْﻟَﻰ ﺑِﻤُﻮﺳَﻰ ﻣِﻨْﻬُﻢْ ”
(“I am closer to Moses than they.”)
So, he observed the fast (on that day) and ordered the Muslims to fast on it.[Sahih al-Bukhari 3397]
Also, Abdullah ibn ‘Abbas (may Allah be pleased with them both) said: “When the Messenger of Allah (peace and blessings of Allah be upon him) fasted on ‘Ashura’ and commanded the Muslims to fast as well, they said, ‘O Messenger of Allah, it is a day that is venerated by the Jews and Christians.’ The Messenger of Allah (peace and blessings of Allah be upon him) said, ‘If I live to see the next year, in sha Allah, we will fast on the ninth day too.’ But it so happened that the Messenger of Allah (peace and blessings of Allah be upon him) passed away before the next year came.” [Reported by Muslim, 1916]
Ash-Shafi’i and his companions, Ahmad, Ishaq and others said: “It is mustahabb to fast on both the ninth and tenth days, because the Prophet (peace and blessings of Allah be upon him) fasted on the tenth, and intended to fast on the ninth.”
On this basis it may be said that there are varying degrees of fasting ‘Ashura’, the least of which is to fast only on the tenth and the best of which is to fast the ninth as well. The more one fasts in Muharram, the better it is.