مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3140
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn ‘Abbas (رضي الله عنه) said: When the Messenger of Allah (ﷺ) came to Madinah, he saw the Jews fasting on the day of 'Ashoora'. He asked they about that and they said: This is the day on which Moosa prevailed over Pharaoh. The Prophet (ﷺ) said to his Companions: "You are closer to Moosa than them, so fast (on this day)."
Musnad Ahmad 3164lts isnad is saheeh, al-Bukhari (4680) and Muslim (1130)]