It was narrated that Ibn 'Abbas said: "The Prophet P.B.U.H 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 P.B.U.H said: 'We have more right to Musa than you do: So he fasted (that day) and enjoined (others) to fast it also:" (Sahih)