It was narrated that Ibn ‘Abbas said:
“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.’”

حَدَّثَنَا سَهْلُ بْنُ أَبِي سَهْلٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ أَيُّوبَ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَدِمَ النَّبِيُّ ـ ﷺ ـ الْمَدِينَةَ فَوَجَدَ الْيَهُودَ صُيَّامًا فَقَالَ " مَا هَذَا " . قَالُوا هَذَا يَوْمٌ أَنْجَى اللَّهُ فِيهِ مُوسَى وَأَغْرَقَ فِيهِ فِرْعَوْنَ فَصَامَهُ مُوسَى شُكْرًا . فَقَالَ رَسُولُ اللَّهِ ـ ﷺ ـ " نَحْنُ أَحَقُّ بِمُوسَى مِنْكُمْ " . فَصَامَهُ وَأَمَرَ بِصِيَامِهِ .