The Prophet ﷺ observed the fast on the 10th of Muharram ('Ashura), and ordered (Muslims) to fast on that day, but when the fasting of the month of Ramadan was prescribed, the fasting of the 'Ashura' was abandoned. ʿAbdullah did not use to fast on that day unless it coincided with his routine fasting by chance.
صَامَ النَّبِيُّ ﷺ عَاشُورَاءَ وَأَمَرَ بِصِيَامِهِ فَلَمَّا فُرِضَ رَمَضَانُ تُرِكَ وَكَانَ عَبْدُ اللَّهِ لاَ يَصُومُهُ إِلاَّ أَنْ يُوَافِقَ صَوْمَهُ