“The Prophet (ﷺ) came to
Al-
Madinah, and he found the Jews observing a fast. He said: ‘What
is
this?’ They said: ‘This is the day when Allah saved Musa and
drowned
Pharaoh, so Musa fasted this day in gratitude.’ The
Messenger of Allah
(ﷺ) said: ‘We have more right to Musa than you
do.’ So he fasted
(that day) and enjoined (others) to fast it
also.’”
Sunan Ibn Majah Book 7, Hadith 97