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.
دَخَلَ النَّبِيُّ ﷺ الْمَدِينَةَ وَإِذَا أُنَاسٌ مِنَ الْيَهُودِ يُعَظِّمُونَ عَاشُورَاءَ وَيَصُومُونَهُ فَقَالَ النَّبِيُّ ﷺ نَحْنُ أَحَقُّ بِصَوْمِهِ فَأَمَرَ بِصَوْمِهِ