When the Prophet ﷺ on the day of 'Ashurah and commanded us to fast on it, they (i.e. Companions) said: Messenger of Allah, this is a day which is considered great by Jews and Christians ? The Messenger of Allah ﷺ said: When the next year comes, we shall fast on the 9th of Muharram. But the next year the Messenger of Allah ﷺ breathed his last.  

سَمِعْتُ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ يَقُولُ حِينَ صَامَ النَّبِيُّ ﷺ يَوْمَ عَاشُورَاءَ وَأَمَرَنَا بِصِيَامِهِ قَالُوا يَا رَسُولَ اللَّهِ إِنَّهُ يَوْمٌ تُعَظِّمُهُ الْيَهُودُ وَالنَّصَارَى فَقَالَ رَسُولُ اللَّهِ ﷺ فَإِذَا كَانَ الْعَامُ الْمُقْبِلُ صُمْنَا يَوْمَ التَّاسِعِ فَلَمْ يَأْتِ الْعَامُ الْمُقْبِلُ حَتَّى تُوُفِّيَ رَسُولُ اللَّهِ ﷺ  

I came to Ibn 'Abbas who was leaning against his sheet of cloth in the Sacred Mosque (al-Masjid al-Haram). I asked him about fasting on the day of 'Ashurah. He said: When you sight the moon of al-Muharram, count (the days). When the 9th of Muharram comes, fast from the morning. I said: Would Muhammad ﷺ observe this fast ? He replied: Thus Muhammad ﷺ used to fast.  

إِذَا رَأَيْتَ هِلاَلَ الْمُحَرَّمِ فَاعْدُدْ فَإِذَا كَانَ يَوْمُ التَّاسِعِ فَأَصْبِحْ صَائِمًا فَقُلْتُ كَذَا كَانَ مُحَمَّدٌ ﷺ يَصُومُ فَقَالَ كَذَلِكَ كَانَ مُحَمَّدٌ ﷺ يَصُومُ