The wisdom behind the Prophet's fasting the tenth of Muharram
15-12-2009 | IslamWeb
Question:
Why did the Prophet, sallallaahu ‘alayhi wa sallam, observe fasting on the tenth of Muharram?
Answer:
All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, would fast certain days apart from those in the month of Ramadan and among these days was the tenth of Muharram. Mu‘aawiyah, may Allaah be pleased with him, said: "I heard the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, saying: 'This is the tenth of Muharram and fasting it is not obligatory for you, but I am fasting, so whoever wants to fast, he may fast and any one who does not want to fast may do so.’" [Al-Bukhari and Muslim]
The Prophet, sallallaahu ‘alayhi wa sallam, observed fasting on this day because Moosa 
would observe fasting on it as this is the day when Allaah saved him (i.e., Moosa) from Fir`awn (Pharaoh). Ibn ‘Abbaas, 
said, "When the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, came to Madeenah, he found the Jews fasting the tenth of Muharram (‘Aashooraa). He, sallallaahu ‘alayhi wa sallam, asked about this and they said, 'It is a good day on which Allaah saved Moosa (Moses) and (his people) the children of Israel from their enemy (Pharaoh) and Moosa used to fast it.‘ Upon this the Prophet, sallallaahu ‘alayhi wa sallam, said: 'I am worthier of Moosa than you.' He, sallallaahu ‘alayhi wa sallam, thus observed fasting and commanded people to observe fasting on this day." [Al-Bukhari and Muslim]
Abu Moosa Al-Ash‘ari, may Allaah be pleased with him, said, "The Day of ‘Aashooraa’ was glorified by the Jews and regarded as a feast. The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: 'You (Muslims) should fast this day.'" [Al-Bukhari and Muslim]
Allaah Knows best.