Riyad as-Salihin
Riyad as-Salihin 1251
The Excellence of Observing Saum on the Day of 'Arafah, 'Ashura' and Tasu'a (i.e., 9th of Muharram)
Arabic hadith text
وعن ابن عباس رضي الله عنهما أن رسول الله صلى الله عليه وسلم صام يوم عاشوراء، وأمر بصيامه. ((متفق عليه))
Translation
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].