وعن ابن عباس رضي الله عنهما
أن رسول الله صلى الله عليه وسلم صام يوم عاشوراء، وأمر بصيامه
The Messenger of Allah (sallallahu alayhi wa sallam) observed Saum (fasting) on the day of Ashura and commanded us to fast on this day.
Riyad as-Salihin 1251, Book 9, The Book of Virtues, Hadith 261. 1 min read