What is the (significance) of this day that you observe fast on it? They said: It is the day of great (significance) when Allah delivered Moses and his people, and drowned the Pharaoh and his people, and Moses observed fast out of gratitude and we also observe it. Upon this the Messenger of Allah ﷺ said: We have more right, and we have a closer connection with Moses than you have; so Messenger of Allah ﷺ observed fast (on the day of 'Ashura), and gave orders that it should be observed. (Using translation from Muslim 1130c)   

أَنَّ رَسُولَ اللهِ ﷺ قَدِمَ الْمَدِينَةَ فَوَجَدَ الْيَهُودَ صِيَامًا يَوْمَ عَاشُورَاءَ فَقَالَ لَهُمْ رَسُولُ اللهِ ﷺ مَا هَذَا الْيَوْمَ الَّذِي تَصُومُونَهُ فَقَالُوا هَذَا يَوْمٌ عَظِيمٌ أَنْجَى الله ﷻ فِيهِ مُوسَى وَقَوْمَهُ وَغَرِقَ فِيهِ فِرْعَوْنُ وَقَوْمُهُ فَصَامَهُ مُوسَى شُكْرًا فَنَحْنُ نَصُومُهُ فَقَالَ رَسُولُ اللهِ ﷺ فَنَحْنُ أَحَقُّ وَأَوْلَى بِمُوسَى مِنْكُمْ فَصَامَهُ رَسُولُ اللهِ ﷺ وَأَمَرَ بِصِيَامِهِ  

[Machine] "He said that he heard Ibn Abbas say, 'I never saw the Messenger of Allah ﷺ eagerly seek to fast a day seeking its virtue over others except for this day, the day of Ashura and the month of Ramadan.'"  

أَنَّهُ سَمِعَ ابْنَ عَبَّاسٍ يَقُولُ مَا رَأَيْتُ رَسُولَ اللهِ ﷺ يَتَحَرَّى صِيَامَ يَوْمٍ يَلْتَمِسُ فَضْلَهُ عَلَى غَيْرِهِ إِلَّا هَذَا الْيَوْمَ يَوْمَ عَاشُورَاءَ وَشَهْرَ رَمَضَانَ  

[Machine] A Bedouin asked the Messenger of Allah ﷺ about his fasting, so the Prophet ﷺ became angry until it was evident on his face. Then 'Umar ibn al-Khattab stood up and said, "We are pleased with Allah as our Lord, Islam as our religion, and you as our Prophet. I seek refuge in Allah from the anger of Allah and the anger of His Messenger." 'Umar continued repeating this until he calmed down. Then he said, "O Messenger of Allah, what do you say about a man who fasts the entire year?" The Prophet ﷺ said, "He neither fasts nor breaks his fast." 'Umar asked, "What about someone who fasts two days and breaks his fast for one day?" The Prophet ﷺ replied, "Whoever is capable of doing so may do it." 'Umar asked again, "And what about someone who breaks his fast for two days and fasts one day?" The Prophet ﷺ said, "I wish that I were given the ability to do that." 'Umar asked, "O Messenger of Allah, what do you say about fasting on Mondays?" The Prophet ﷺ said, "That is the day I was born and the day on which revelation was sent down to me." 'Umar asked, "O Messenger of Allah, what do you say about fasting for one day and breaking the fast on the next day?" The Prophet ﷺ said, "That is the fasting of my brother Dawud (David), ﷺ ." 'Umar asked, "O Messenger of Allah, what do you say about fasting on the day of 'Ashura'?" The Prophet ﷺ said, "Indeed, I hope that Allah will forgive the sins of the previous year." 'Umar asked, "O Messenger of Allah, what do you say about fasting on the day of Arafah?" The Prophet ﷺ said, "Indeed, I hope that Allah will forgive the sins of the previous and upcoming year."  

أَنَّ أَعْرَابِيًّا سَأَلَ رَسُولَ اللهِ ﷺ عَنْ صَوْمِهِ فَغَضِبَ حَتَّى عُرِفَ ذَلِكَ فِي وَجْهِهِ فَقَامَ عُمَرُ بْنُ الْخَطَّابِ ؓ فَقَالَ رَضِينَا بِاللهِ رَبًّا وَبِالْإِسْلَامِ دِينًا وَبِكَ نَبِيًّا أَعُوذُ بِاللهِ مِنْ غَضَبِ اللهِ وَغَضَبِ رَسُولِهِ فَلَمْ يَزَلْ عُمَرُ ؓ يُرَدِّدُ ذَلِكَ حَتَّى سَكَنَ فَقَالَ يَا رَسُولَ اللهِ مَا تَقُولُ فِي رَجُلٍ يَصُومُ الدَّهْرَ كُلَّهُ فَقَالَ رَسُولُ اللهِ ﷺ لَا صَامَ وَلَا أَفْطَرَ أَوْ قَالَ مَا صَامَ وَمَا أَفْطَرَ فَقَالَ يَا رَسُولَ اللهِ كَيْفَ بِمَنْ يصُومُ يَوْمَيْنِ وَيُفْطِرُ يَوْمًا فَقَالَ وَمَنْ يَطِيقُ ذَلِكَ فَقَالَ يَا رَسُولَ اللهِ كَيْفَ بِمَنْ يفْطِرُ يَوْمَيْنِ وَيَصُومُ يَوْمًا فَقَالَ لَوَدِدْتُ أَنِّي طُوِّقْتُ ذَلِكَ فَقَالَ يَا رَسُولَ اللهِ فَمَا تَقُولُ فِي صَوْمِ يَوْمِ الِاثْنَيْنِ فَقَالَ ذَلِكَ يَوْمٌ وُلِدْتُ فِيهِ وَأُنْزِلَ عَلَيَّ فِيهِ فَقَالَ يَا رَسُولَ اللهِ فَمَا تَقُولُ فِي رَجُلٍ يَصُومُ يَوْمًا وَيُفْطِرُ يَوْمًا فَقَالَ ذَلِكَ صَوْمُ أَخِي دَاوُدَ عَلَيْهِ السَّلَامُ قَالَ يَا رَسُولَ اللهِ فَمَا تَقُولُ فِي صَوْمِ يَوْمِ عَاشُورَاءَ قَالَ إِنِّي لَأَحْتَسِبُ عَلَى اللهِ ﷻ أَنْ يُكَفِّرَ السَّنَةَ قَالَ يَا رَسُولَ اللهِ فَمَا تَقُولُ فِي صَوْمِ يَوْمِ عَرَفَةَ قَالَ إِنِّي لَأَحْتَسِبُ عَلَى اللهِ أَنْ يُكَفِّرَ السَّنَةَ الَّتِي قَبْلَهَا وَالسَّنَةَ الَّتِي بَعْدَهَا  

[Machine] I saw someone ordering the fasting on the day of Ashura from Ali and Abu Musa.  

رَأَيْتُ أَحَدًا كَانَ آمَرَ بِصِيَامِ يَوْمِ عَاشُورَاءَ مِنْ عَلِيٍّ وَأَبِي مُوسَى ؓ