Ibn 'Abbas (May Allah be pleased with them) reported:
The Messenger of Allah (ﷺ) observed Saum (fasting) on the day of 'Ashura' and commanded us to fast on this day.[Al-Bukhari and Muslim]. (DE)
App reference (DE): Book 9, Hadith 261 (DE)
Arabic/English book reference (DE): Book 9, Hadith 1251 (DE)