Ibn Majah

1؎: The tenth day of Muharram is called the Day of Ashura. When the Prophet sallallahu alayhi wa sallam migrated from Makkah and arrived in Madinah, he saw that the Jews were fasting on this day. He sallallahu alayhi wa sallam asked them: "Why do you fast on this day?" They replied that on this day Allah Ta'ala granted Musa alayhis salam deliverance from Pharaoh, and in celebration of this, they fast. The Prophet sallallahu alayhi wa sallam said: "We have a greater right to Musa than you." Accordingly, he sallallahu alayhi wa sallam fasted on this day, and also said: "If I remain alive till next year, I will also fast on the 9th of Muharram along with it," so that there would be opposition to the Jews. In fact, in one narration, he sallallahu alayhi wa sallam also commanded: "Fast on the Day of Ashura, and oppose the Jews; fast a day before or after it as well."