(20) باب أَىُّ يَوْمٍ يُصَامُ فِي عَاشُورَاءَ
Hakam b. al-'Araj reported:
I went to Ibn 'Abbas (Allah be Pleased with both of them) and he was reclining using his mantle as a pillow near the fountain of Zamzam. I said to him: Tell me about fasting on Ashura. He said: When you see the new moon of Muharram then count the (days) and observe fast on the 9th. I said to him: Is it how the Messenger of Allah (ﷺ) observed the fast? He said: Yes.
Reference: Sahih Muslim 1133 a
In-book reference: Book 13, Hadith 170
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2526