(69) باب صِيَامِ يَوْمِ عَاشُورَاءَ
Narrated Ibn `Abbas:
The Prophet (ﷺ) came to Medina and saw the Jews fasting on the day of Ashura. He asked them about that. They replied, "This is a good day, the day on which Allah rescued Bani Israel from their enemy. So, Moses fasted this day." The Prophet (ﷺ) said, "We have more claim over Moses than you." So, the Prophet fasted on that day and ordered (the Muslims) to fast (on that day).
Reference: Sahih al-Bukhari 2004
In-book reference: Book 30, Hadith 109
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 222