رِيَاضُ الصَّالِحِينَ
Riyad as-Salihin 1251
The Book of Virtues
The Excellence of Observing Saum on the Day of 'Arafah, 'Ashura' and Tasu'a (i.e., 9th of Muharram)
وَعَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّه عَلَيْهِ وَسلم صام يَوْمَ عَاشُورَاءَ، وَأَمَرَ بِصِيَامِهِ. ((مُتَّفَقٌ عَلَيْهِ))
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].
Riyad as-Salihin, 1251