It wasnarrated that Ibn 'Abbas said:
"The Prophet (ﷺ) came toAl-Madinah, and he found the Jews observing a fast. He said: 'Whatisthis?' They said: 'This is the day when Allah saved Musa anddrownedPharaoh, so Musa fasted this day in gratitude.' TheMessenger of Allah(ﷺ) said: 'We have more right to Musa than youdo.' So he fasted(that day) and enjoined (others) to fast italso.'"
English reference: Vol. 1, Book 7, Hadith 1734
Arabic reference: Book 7, Hadith 1806