"I met up with Ibn Abbas while he was reclining on his Rida at the Zamzam (well). So I said: 'Inform me about the Day of Ashura. On which day is it fasted?' He said: 'When you see the crescent of Al-Muharram, then count, then fast on the morning of the ninth day.'" He said: 'I said: Is this how Muhammad fasted it?' He said: 'Yes.'"  

أَخْبِرْنِي عَنْ يَوْمِ عَاشُورَاءَ أَىُّ يَوْمٍ هُوَ أَصُومُهُ فَقَالَ إِذَا رَأَيْتَ هِلاَلَ الْمُحَرَّمِ فَاعْدُدْ ثُمَّ أَصْبِحْ مِنَ التَّاسِعِ صَائِمًا قَالَ فَقُلْتُ أَهَكَذَا كَانَ يَصُومُهُ مُحَمَّدٌ ﷺ قَالَ نَعَمْ  

"The Messenger of Allah ordered fasting the tenth day for the Day of Ashura."  

أَمَرَ رَسُولُ اللَّهِ ﷺ بِصَوْمِ عَاشُورَاءَ يَوْمَ الْعَاشِرِ