All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The Hadeeth mentions that Prophet Moosa (Moses) observed fasting on the Day of ‘Ashooraa’ (the tenth of Muharram) as an expression of gratitude to Allaah and we know of no other reason for which he fasted on that day.
The Hadeeth was reported in the Musnad of Imam Ahmad with the following wording: "...and this is the day when the ship (of Prophet Nooh [Noah]) came to rest on (Mount) Judi, so Nooh and Moosa fasted on it to show gratitude to Allaah." Shaykh Shu‘ayb Al-Arnaa'oot classified its chain of narrators as weak in his revision of the Musnad.
For more benefit, please refer to Fatwa 276160.
Allaah Knows best.