Hafiz Abdul Mannan An-Nurpuri (rah) wrote as quoted in his “Ahkam Masail” v 1 p 285 that fasting of Ashura has been changed to 9th of Muharram:

“It is narrated in Sahih Muslim that the Messenger of Allah (saw) said that if he remains alive the next year, he will fast on the 9th (of Muharram), so if anyone wants to fast only one day for Ashura, he should fast on the 9th and not 10th as the Prophet (saw) changed the date of 10th to 9th. Allah knows best” End of Hafiz An-Nurpuri’s words

The Hadith is reported in Sahih Muslim, Eng tran. v 3 p 206, Book of Fasting, chapter 20, Which day should be fasted for Ashura?

N 2666 which says: Abu Ghatafan bin Tarif Al-Murri said: “I heard ‘Abdullah bin Abbas (may Allah be pleased with them) say: “When the Messenger of Allah (saw) fasted on the day of Ashura and enjoined this fast, they said: “O Messenger of Allah, it is a day that is venerated by the Jews and Christians.” The Messenger of Allah (saw) said: “Next year, if Allah wills, we will fast on the ninth day.” He said: “But the next year the Messenger of Allah (saw) had passed away.”

N 2667: “It was narrated that ‘Abdullah bin ‘Abbas (may Allah be pleased with them) said: “The Messenger of Allah (saw) said: “If I live until next year, I will certainly fast on the ninth day”

Hafiz An-Nurpuri also wrote as on p 286 of the same vol.:

“The Fatwa of ‘Abdullah ibn ‘Abbas of fasting on the 9th (of Muharram) is according to Marfu’ Hadith, rather the words of the Hadith in Sahih Muslim is a clear Nass that he gave the Fatwa of the 9th based on a Marfu’ Hadith, while his Fatwa for fasting on 9th and 10th, there is not Marfu’ Sahih Hadith supporting it, rather this Fatwa is opposing the Prophetic Hadith of the 9th, this is why his Fatwa of 9th is Rajih (preponderant).

So the Fatwa (of ibn ‘Abbas) of fasting on 9th is in “Sahih Muslim” and the Fatwa of fasting on the 9th and 10th is in “Musannaf AbdurRazzaq” and the Muhadiths have made in their principles of Tarjeeh (giving preponderance) that at time of contradiction, the Ahadith of Sahih Bukhari and Sahih Muslim will have Tarjeeh over other books of Hadith, and this is why his Fatwa of 9th is Rajih.

And except the Fatawa of fasting on 9th, all others Fatawa are against the Marfu’ Sahih Hadith, and when a Fatwa opposes a Marfu’ Hadith, even if it is the narrator of the Hadith who gives such a Fatwa, then action will be according to the Marfu’ Hadith and not on the Fatwa, as there is no proof for anyone with the Prophet (saw) (La Hujjata li Ahadin ma’a Nadbi (saw)), and there is very good discussion on this topic in “Irshad Al-Fuhul” (of Imam Ash-Shawkani) and you can check it”

End of Hafiz An-Nurpuri’s words

The Fatwa of ibn ‘Abbas of fasting on the 9th is in “Sahih Muslim” Eng tran. v 3 p 205, Book of Fasting, chapter 20, Which day should be fasted for Ashura? N 2664:

“It was narrated that Al-Hakam bin Al-A’raj said: “I came to ibn ‘Abbas (may Allah be pleased with them) while he was reclining on his Rida at Zamzam and said to him: “Tell me about the fast of Ashura.” He said: “When you see the crescent of Muharram, then count, and fast on the ninth day.” I said: “Is this how the Messenger of Allah (saw) used to fast it?” He said: “Yes””

It is apparent that in this Fatwa, Ibn ‘Abbas said to fast on the 9th and his saying: “Yes” means: “Yes, this is how the Prophet (saw) would have fasted if he remained alive”, and in this Fatwa of Sahih Muslim, ibn ‘Abbas did not say to fast on the 9th and 10th and did not say that the Prophet (saw) would have fasted on 9th and 10th.

Imam At-Tirmidhi mentioned in chapter 50. What has been related about which day Ashura is, translation Darussalam:

754 “It was narrated that Al-Hakam bin Al-A’raj said: “I came to ibn ‘Abbas (may Allah be pleased with them) while he was reclining on his Rida at Zamzam and said to him: “Tell me about the fast of Ashura.” He said: “When you see the crescent of Muharram, then count, and fast on the ninth day.” I said: “Is this how the Messenger of Allah (saw) used to fast it?” He said: “Yes”” (It is also narrated by Muslim)

755: Ibn ‘Abbas narrated: “The Messenger of Allah (saw) ordered fasting the tenth day for the Day of Ashura (Da’if). Abu Eisa said: The Hadith of ibn ‘Abbas is a Hasan Sahih Hadith.

The people of knowledge differ over the day of Ashura, some of them say it is the ninth day, some of them said it is the tenth day. It has been reported that ibn ‘Abbas said: “Fast the ninth and the tenth, and differ from the Jews.” Ash-Shafi’i, Ahmad and Ishaq followed this Hadith”

End of Imam At-Tirmidhi’s words

The narration of ibn ‘Abbas: “Fast the ninth and the tenth, and differ from the Jews” is reported in “Musannaf AburRazaq” and others and is authentic, but it is opposing his Fatwa of fating on 9 in “Sahih Muslim”.

As for Hadith 755 in Sunnan At-Tirmidhi, sh Zubayr Ali Zai weakened it because of Tadlees of Hasan, but even if it is Sahih, it would be for the years before, as the Prophet (saw) only said he would next year fast on 9th, so changing 10th to 9th was the last matter, and order to fast on 10th which appears in other Ahadith was before. Allah knows best.

May Allah send Salah and Salam on the Prophet (saw), his family and companions!