I went to Ibn 'Abbas (Allah be Pleased with both of them) and he was reclining using his mantle as a pillow near the fountain of Zamzam. I said to him: Tell me about fasting on Ashura. He said: When you see the new moon of Muharram then count the (days) and observe fast on the 9th. I said to him: Is it how the Messenger of Allah ﷺ observed the fast? He said: Yes.  

إِذَا رَأَيْتَ هِلاَلَ الْمُحَرَّمِ فَاعْدُدْ وَأَصْبِحْ يَوْمَ التَّاسِعِ صَائِمًا قُلْتُ هَكَذَا كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُهُ قَالَ نَعَمْ  

I asked Ibn Abbas (Allah be pleased with them) as he was reclining using his cloak as a pillow near Zamzam about fasting on 'Ashura. The rest of the hadith is the same.  

عَنْ صَوْمِ عَاشُورَاءَ بِمِثْلِ حَدِيثِ حَاجِبِ بْنِ عُمَرَ  

Messenger of Allah, it is a day which the Jews and Christians hold in high esteem. Thereupon the Messenger of Allah ﷺ said: When the next year comes, God willing, we would observe fast on the 9th But the Messenger of Allah ﷺ died before the advent of the next year.  

حِينَ صَامَ رَسُولُ اللَّهِ ﷺ يَوْمَ عَاشُورَاءَ وَأَمَرَ بِصِيَامِهِ قَالُوا يَا رَسُولَ اللَّهِ إِنَّهُ يَوْمٌ تُعَظِّمُهُ الْيَهُودُ وَالنَّصَارَى فَقَالَ رَسُولُ اللَّهِ ﷺ فَإِذَا كَانَ الْعَامُ الْمُقْبِلُ إِنْ شَاءَ اللَّهُ صُمْنَا الْيَوْمَ التَّاسِعَ قَالَ فَلَمْ يَأْتِ الْعَامُ الْمُقْبِلُ حَتَّى تُوُفِّيَ رَسُولُ اللَّهِ ﷺ  

If I live till the next (year), I would definitely observe fast on the 9th, and the narration transmitted by Abu Bakr is:" He meant the day of Ashura."  

قَالَ رَسُولُ اللَّهِ ﷺ لَئِنْ بَقِيتُ إِلَى قَابِلٍ لأَصُومَنَّ التَّاسِعَ وَفِي رِوَايَةِ أَبِي بَكْرٍ قَالَ يَعْنِي يَوْمَ عَاشُورَاءَ