Observing Saum (fast) on the day of 'Ashura.
حَدَّثَنَا أَبُو مَعْمَرٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا أَيُّوبُ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ سَعِيدِ بْنِ جُبَيْرٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ قَدِمَ النَّبِيُّ صلى الله عليه وسلم الْمَدِينَةَ، فَرَأَى الْيَهُودَ تَصُومُ يَوْمَ عَاشُورَاءَ، فَقَالَ " مَا هَذَا ". قَالُوا هَذَا يَوْمٌ صَالِحٌ، هَذَا يَوْمٌ نَجَّى اللَّهُ بَنِي إِسْرَائِيلَ مِنْ عَدُوِّهِمْ، فَصَامَهُ مُوسَى. قَالَ " فَأَنَا أَحَقُّ بِمُوسَى مِنْكُمْ ". فَصَامَهُ وَأَمَرَ بِصِيَامِهِ.
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).