(52) باب إِتْيَانِ الْيَهُودِ النَّبِيَّ صلى الله عليه وسلم حِينَ قَدِمَ الْمَدِينَةَ
Narrated Ibn `Abbas:
When the Prophet (ﷺ) arrived at Medina he found that the Jews observed fast on the day of 'Ashura'. They were asked the reason for the fast. They replied, "This is the day when Allah caused Moses and the children of Israel to have victory over Pharaoh, so we fast on this day as a sign of glorifying it." Allah's Messenger (ﷺ) said, "We are closer to Moses than you." Then he ordered that fasting on this day should be observed.
Reference: Sahih al-Bukhari 3943
In-book reference: Book 63, Hadith 168
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 58, Hadith 279