Sahih Muslim Book - 13, Hadith - 2656

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا هُشَيْمٌ عَنْ أَبِى بِشْرٍ عَنْ سَعِيدِ بْنِ جُبَيْرٍ عَنِ ابْنِ عَبَّاسٍ - رضى الله عنهما - قَالَ قَدِمَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- الْمَدِينَةَ فَوَجَدَ الْيَهُودَ يَصُومُونَ يَوْمَ عَاشُورَاءَ فَسُئِلُوا عَنْ ذَلِكَ فَقَالُوا هَذَا الْيَوْمُ الَّذِى أَظْهَرَ اللَّهُ فِيهِ مُوسَى وَبَنِى إِسْرَائِيلَ عَلَى فِرْعَوْنَ فَنَحْنُ نَصُومُهُ تَعْظِيمًا لَهُ. فَقَالَ النَّبِىُّ -صلى الله عليه وسلم- « نَحْنُ أَوْلَى بِمُوسَى مِنْكُمْ ». فَأَمَرَ بِصَوْمِهِ.

It was narrated that Ibn Abbas [may Allah be pleased with them] said: “The Messenger of Allah came to Al-Madinah, and he found the Jews fasting on the day of Ashura. They were asked about that and they said: ‘This is the day on which Allah granted victory to Musa and the Children of Israel over Firawn (Pharaoh). We fast on this day out of reverence for it. The Prophet said: ‘We are closer to Musa than you, and he enjoined fasting on that day.”