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