مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2150
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Dawood bin 'Ali, from his father, that his grandfather Ibn 'Abbas said: The Messenger of Allah (ﷺ) said: “Fast the day of ‘Ashoora' but be different from the Jews; fast one day before it or one day after it."
Musnad Ahmad 2154lts isnad is da'eef]