It is narrated from Ibn Umar (may Allah be pleased with them both) that he stated: The fast of the day of Ashura was mentioned to the Messenger of Allah (ﷺ), so the Prophet (ﷺ) said: "The people of ignorance used to fast on this day. Whoever among you wishes to fast on this day may do so, and whoever dislikes it may leave it."
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 632
Hadith Takhrijاخرجه البخارى، التفسير باب يايها الذين امنوا كتب عليكم الصيام كما كتب...... الخ ) (4501) ومسلم ، الصيام، باب صوم يوم عاشوراء (1126)۔