Sahih Muslim Book - 13, Hadith - 2658

وَحَدَّثَنِى ابْنُ أَبِى عُمَرَ حَدَّثَنَا سُفْيَانُ عَنْ أَيُّوبَ عَنْ عَبْدِ اللَّهِ بْنِ سَعِيدِ بْنِ جُبَيْرٍ عَنْ أَبِيهِ عَنِ ابْنِ عَبَّاسٍ - رضى الله عنهما - أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- قَدِمَ الْمَدِينَةَ فَوَجَدَ الْيَهُودَ صِيَامًا يَوْمَ عَاشُورَاءَ فَقَالَ لَهُمْ رَسُولُ اللَّهِ -صلى الله عليه وسلم- « مَا هَذَا الْيَوْمُ الَّذِى تَصُومُونَهُ ». فَقَالُوا هَذَا يَوْمٌ عَظِيمٌ أَنْجَى اللَّهُ فِيهِ مُوسَى وَقَوْمَهُ وَغَرَّقَ فِرْعَوْنَ وَقَوْمَهُ فَصَامَهُ مُوسَى شُكْرًا فَنَحْنُ نَصُومُهُ. فَقَالَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- « فَنَحْنُ أَحَقُّ وَأَوْلَى بِمُوسَى مِنْكُمْ ». فَصَامَهُ رَسُولُ اللَّهِ -صلى الله عليه وسلم- وَأَمَرَ بِصِيَامِهِ.

It was narrated from Ibn Abbas [may Allah be pleased with them] that the Messenger of Allah came to Al-Madinah and found the Jews fasting on the day of Ashura. The Messenger of Allah said to them: “What is this day that you are fasting?” They said: “This is a great day, on which Allah saved Musa and his people, and drowned Firawn (Pharaoh) and his people. Musa fasted on this day out of gratitude and we fast it too,” The Messenger of Allah said: “We are more entitled to be closer to Musa than you,” and the Messenger of Allah fasted (on this day) and enjoined fasting on it.