Is the statement attributed to Ibn Abbas concerning the day of Ashura, that it is the ninth of Muharram and that the Prophet, peace and blessings be upon him, used to fast on the ninth, from the words of Imam Muslim, or is it attributed to someone else?

The phrases mentioned concerning the Day of Ashura have not been found with this exact wording in the versions of Sahih Muslim or other hadith books. They might be from Imam Muslim's own words in an unavailable version, or from Imam Al-Nawawi summarizing the opinion of Ibn Abbas. It appears that Ibn Abbas believed the Day of Ashura to be the ninth day of Muharram, basing his view on what Muslim narrated from him. However, the majority of scholars hold that the Day of Ashura is the tenth day of Muharram.