Riyad as-Salihin
Riyad as-Salihin 1253
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 قال: قال رسول الله صلى الله عليه وسلم: "لئن بقيت إلى قابل لأصومن التاسع" ((رواه مسلم)).
Translation
Ibn 'Abbas (May Allah be pleased with them) reported
The Messenger of Allah (ﷺ) said, "If I remain alive till the next year, I shall also observe fast on the ninth of Muharram."[Muslim].