Al-Hakam b. al-A'raj said: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

English translation · Sunan Abu Dawud