مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3086
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) came to Madinah and found the Jews fasting on the day of ‘Ashoora". He said: "What is this?" They said: This is a great day, the day on which Allah saved Moosa and drowned the people of Pharaoh, so Moosa fasted it in gratitude. The Prophet (ﷺ) said: “Verily I am closer to Moosa and have more right to fast it.” So he fasted it and instructed (the Muslims) to fast it.
Musnad Ahmad 3112Its isnad is saheeh according to the conditions of al-Bukhar