Did the Messenger (blessings and peace of Allah be upon him) not come to know that the Jews fasted on ‘Ashoora’ except in his final year, because he (blessings and peace of Allah be upon him) said: “If I live until next year, I will surely fast the ninth day [of Muharram] too”? (Narrated by Muslim).

Praise
be to Allah

The Messenger (blessings and peace of
Allah be upon him) fasted ‘Ashoora’ the first year he came to Madinah,
and he continued to do so. Then in the last year of his life, he
(blessings and peace of Allah be upon him) wanted to differ from the
Jews by fasting the ninth day along with the tenth. The hadiths that
were narrated about fasting ‘Ashoora’ indicate that.

It was narrated that Ibn ‘Abbaas (may
Allah be pleased with him) said: The Prophet (blessings and peace of Allah
be upon him) came to Madinah and saw the Jews fasting on the day of
‘Ashoora. He said: “What is this?” They said: This is a good day; it is the
day on which Allah saved the Children of Israel from their enemy, so Moosaa
fasted on this day. He said: “We have more right to Moosaa than you.” So he
fasted on this day and issued instructions that a fast was to be observed on
this day. Narrated by al-Bukhaari (2004).

It was also narrated that Ibn ‘Abbaas (may
Allah be pleased with him) said: When the Messenger of Allah (blessings and
peace of Allah be upon him) fasted on the day of ‘Ashoora’, and issued
instructions that a fast was to be observed on this day, they said: O
Messenger of Allah, it is a day that is venerated by the Jews and
Christians. The Messenger of Allah (blessings and peace of Allah be upon
him) said: “Next year, if Allah wills, we will fast on the ninth day too.”
But before the next year came, the Messenger of Allah (blessings and peace
of Allah be upon him) passed away. Narrated by Muslim (1134)

Ibn al-Qayyim (may Allah have mercy on
him) said: This indicates that he fasted on this day and issued instructions
that this fast be observed, one year before he died, and that what he said
earlier about it was when he first came to Madinah.

End quote from Zaad al-Ma‘aad
(2/67)

He also said concerning the second hadith
of Ibn ‘Abbaas (2/81):

Undoubtedly this was at the end of the
matter; in the beginning, he liked to do the same as the People of the Book
in matters concerning which he had received no divine instructions. End
quote.

Shaykh al-Islam Ibn Taymiyah (may Allah
have mercy on him) said:

Then when it was said to him, shortly
before he died, that it was a day that was venerated by the Jews and the
Christians, he gave instructions to differ from them by adding another day
to it, and he resolved to do that.

End quote from Iqtida’ as-Siraat al-Mustaqeem
(1/87)

And Allah knows best.