مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2629
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 saw the Jews fasting on the day of ‘Ashoora". He said: “What is this day on which you fast?” They said: This is a good day; it is the day on which Allah saved the Children of Israel from their enemy, so Moosa fasted on this day. The Messenger of Allah (ﷺ) said: “I am more entitled to (be close to) Moosa than you." So the Messenger of Allah (ﷺ) fasted on that day and enjoined fasting thereon.
Musnad Ahmad 2644Its isnad is saheeh, al-Bukhari (2004) and Muslim (1130)]