When the Prophet (ﷺ) arrived at Medina, he noticed that some people among the Jews used to respect Ashura' (i.e. 10th of Muharram) and fast on it. The Prophet (ﷺ) then said, "We have more right to observe fast on this day." and ordered that fasting should be observed on it.
Sahih al-Bukhari Book 63, Hadith 166
← Previous Hadith The Prophet (ﷺ) said, "Had only ten Jews (amongst their chiefs) believe me, all the Jews would definitely have believed me."

Sahih al-Bukhari Book 63, Hadith 165 Next Hadith → 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.

Sahih al-Bukhari Book 63, Hadith 167