´Al-Hakam bin Al-Arfaj said:` "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.'"
Hadith Referenceسنن ترمذي / كتاب الصيام عن رسول الله صلى الله عليه وسلم / 754
This hadith has been narrated from Abu Juhaim Ansari by another chain of transmitters.
Shaykh Maulana Abdul Aziz Alvi
Hadith Commentary:Benefits and Issues: This hadith proves that passing in front of someone performing prayer is a very grave sin. If a person truly understands the magnitude of this sin, then he would never dare to pass in front of someone praying, even if he has to wait for a long time. Although in some narrations the number mentioned is forty years, and in others, one hundred years.
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 1133
Al-Hakam bin al-Araj 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.
Shaykh Umar Farooq Saeedi
Benefits and Issues: The Messenger of Allah (sallallahu alayhi wa sallam) did not actually fast on the ninth (of Muharram), but his intention was indeed to do so. Based on this, Ibn Abbas (radi Allahu anhu) stated that Muhammad (sallallahu alayhi wa sallam) used to do so. And what is required is also this: that one should fast on the ninth and tenth, or the tenth and eleventh.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2446