Tirmidhi

Hadith Commentary: Benefits and Issues: The Messenger of Allah (sallallahu alayhi wa sallam), in the last year of his life, expressed that if he remained alive, he would also fast on the ninth (9th) day, as will be mentioned ahead in the narration of Ibn Abbas (radi Allahu anhuma). This statement can have two meanings: one, that in the future, instead of fasting on the tenth of Muharram, we should fast on the ninth of Muharram; the second, that from now on, we should fast on both the ninth and the tenth of Muharram, so that there is a distinction between our practice and that of the Jews and Christians, and the resemblance is eliminated. The narration of Musnad Ahmad gives preference to this second interpretation. The Prophet (sallallahu alayhi wa sallam) said: "Fast on the day of Ashura and oppose the Jews, and fast a day before or after it." The majority of the Ummah agrees upon this meaning, although some scholars of this era are of the opinion that, since in our time the Jews and Christians do not conduct any of their affairs according to lunar months, therefore, now there is no question of any sharing or resemblance, and thus, in our time, it is not necessary to fast on the ninth or eleventh to remove resemblance. In the narration of Tahawi, Ibn Abbas (radi Allahu anhuma) said: "Oppose the Jews and fast on both the ninth and the tenth." (Fath al-Mulhim, p. 145, vol. 3)

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.