The Jews used to regard the day of Ashura as a festival. The Prophet ﷺ said, "So you fast it."In another narration, the Prophet ﷺ entered Madinah and found some Jews honoring Ashura and fasting on it. The Prophet ﷺ said, "We have more right to fast it than they do." So he ordered that it be fasted.
كان يوم عاشوراء تعده اليهود عيدا، قال النبي ﷺ: فصوموه أنتم. وفي رواية: دخل النبي ﷺ المدينة، وإذا أناس من اليهود يعظمون عاشوراء ويصومونه، فقال النبي ﷺ: نحن أحق بصومه فأمر بصومه.
Bukhari related it in the Book of Fasting (2005), and Muslim in the Book of Fasting (1131), through Abu Usama (that is, Hammad bin Usama), from Abul-Umays (that is, Utba bin Abdullah Masudi), from Qays bin Muslim, from Tariq bin Shihab, from Abu Musa, who said (then he mentioned the hadith). The second narration is from Bukhari in the Book of Virtues (3942), through Hammad bin Usama with the same chain. Muslim added in one narration, Abu Usama said, "Sadaqa bin Abu Imran narrated to me, from Qays bin Muslim, from Tariq bin Shihab, from Abu Musa, who said, 'The people of Khaybar used to fast on the day of Ashura, taking it as a festival, and they would dress their women in their jewelry and finery on that day. So the Messenger of Allah ﷺ said, "You fast it yourselves."'"His statement "their finery" means their fine appearance.Ibn Hajar said, "Its apparent meaning is that the motive behind commanding its fasting was love of differing from the Jews, so that fasting would be observed on a day when they broke their fast, since the day of a festival is not fasted. The hadith of Ibn Abbas indicates that the motive for fasting it was agreement with them regarding the reason, which was giving thanks to Allah the Exalted for the deliverance of Musa. However, their glorifying it and believing it to be a festival does not necessarily mean they did not fast it. Perhaps fasting it was part of their glorification of it in their law, and this has come explicitly in this hadith of Abu Musa, as extracted by the author in the Book of Migration, with the wording, 'And there were some people of the Jews glorifying Ashura and fasting it.'" End of quote.On this subject there is also what was related from Abu Hurairah, who said, the Prophet ﷺ passed by some people of the Jews who had fasted on the day of Ashura. He said, "What is this fast?" They said, "This is the day on which Allah saved Musa and Banu Israil from drowning, and Pharaoh was drowned on it. And this is the day on which the ship came to rest on Judi, so Nuh and Musa fasted in thanks to Allah." The Prophet said, "I have more right to Musa, and more right to the fasting of this day." So he commanded his companions to fast.Ahmad related it (8717) from Abu Jafar, who said, Abdul-Samad informed us, from his father, from Shubayl, from Abu Hurairah, who mentioned it. Abdul-Samad is Ibn Habib, who is daif, and his father is unknown, as mentioned before. There is also an objectionable element in his statement, "and this is the day on which the ship came to rest on Judi, so Nuh fasted."