مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3370
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that al-Hakam bin al-A'raj said: I asked Ibn ‘Abbas (رضي الله عنه) about the day of 'Ashoora'. He said: When you see the new moon of al-Muharram, count, and when the ninth day comes, fast. Yoonus said: I was told that al-Hakam said: I said: Is that how Muhammad (ﷺ) fasted? He said: Yes.
Musnad Ahmad 3393Its isnad is saheeh]