I came to Ibn Abbas while he was reclining on his cloak near Zamzam, and I said to him, "Tell me about fasting on the day of Ashura."He replied, "When you sight the crescent of Muharram, count the days, and begin fasting on the morning of the ninth."I asked, "Is this how the Messenger of Allah ﷺ used to fast it?"He answered, "Yes."
انتهيت إلى ابن عباس وهو متوسد رداءه في زمزم، فقلت له: أخبرني عن صوم عاشوراء؟ فقال: إذا رأيت هلال المحرم فاعدد، وأصبح يوم التاسع صائما. قلت: هكذا كان رسول الله ﷺ يصومه؟ قال: نعم.
Muslim narrated it in the Book of Fasting (1133) via Hajib bin Umar and Muawiya bin Amr, separately, from Hakam, from Araj, with the same wording, and he related it.His saying, "and he would fast on the ninth day," means along with the tenth, which is the day of Ashura. This is because the questioner knew that Ashura is the tenth day, but his question was, "How should we fast it?" So he answered him accordingly, because he himself was the one who narrated from the Prophet ﷺ, "If next year comes, Allah willing, we will fast the ninth day," and it is to this that he was pointing in his answer to the questioner.As for the questioner's statement, "Is this how the Messenger of Allah ﷺ used to fast it?" and the reply, "Yes," the word "yes" indicates that the Prophet ﷺ had resolved to do so, even though he did not actually do it, so his resolve was treated as if it were an action. Otherwise, it is not established from the Prophet ﷺ that he ever fasted on the ninth day at all.This interpretation is necessary so that the various narrations do not contradict one another, since all of them are sahih. Whatever has been said contrary to this is far removed from both the language and the sharia.