Does not fasting the eleventh day constitute an innovation (bid‘ah) since the Prophet (peace and blessings be upon him) and his Companions did not do so, especially given that he did not say, "If I live until next year, I will fast the ninth and tenth" to differ from the Jews? And is it sufficient to fast the tenth for someone who missed fasting the ninth?

It is recommended to fast the eleventh day of Muharram, based on the hadith: "Fast the Day of Ashura, and differ from the Jews in it; fast a day before it or a day after it." Although scholars have differed on the authenticity of this hadith, such a slight weakness in traditions concerning virtuous deeds is tolerated, especially given the general encouragement to fast in Muharram. Another narration, with the wording "Fast a day before it and a day after it," might suggest the recommendation to fast the ninth, tenth, and eleventh days.

Another reason mentioned is as a precaution for the tenth day, so by fasting the ninth, tenth, and eleventh days, the fasting of Ashura is certainly achieved.

Fasting these three days is not considered an innovation. And there is no harm for someone who missed fasting the ninth and fasted only the tenth; if they add the eleventh, it is better. And it is not disliked to fast only the tenth, according to the sounder opinion.