It was reported that the day the Children of Israel were saved from Fir`awn was called the day of `Ashura'. Imam Ahmad reported that Ibn `Abbas said that the Messenger of Allah came to Al-Madinah and found that the Jews were fasting the day of `Ashura'. He asked them, "What is this day that you fast'' They said, "This is a good day during which Allah saved the Children of Israel from their enemy, and Musa used to fast this day.'' The Messenger of Allah said,
«أَنَا أَحَقُّ بِمُوسَى مِنْكُم»
(I have more right to Musa than you have.)
So the Messenger of Allah fasted that day and ordered that it be fasted. This Hadith was collected by Al-Bukhari, Muslim, An-Nasa'i and Ibn Majah.
﴿وَإِذْ وَعَدْنَا مُوسَى أَرْبَعِينَ لَيْلَةً ثُمَّ اتَّخَذْتُمُ الْعِجْلَ مِن بَعْدِهِ وَأَنتُمْ ظَـلِمُونَ - ثُمَّ عَفَوْنَا عَنكُمِ مِّن بَعْدِ ذَلِكَ لَعَلَّكُمْ تَشْكُرُونَ - وَإِذْ ءَاتَيْنَا مُوسَى الْكِتَـبَ وَالْفُرْقَانَ لَعَلَّكُمْ تَهْتَدُونَ ﴾
(51. And (remember) when We appointed for Musa (Moses) forty nights, and (in his absence) you took the calf (for worship), and you were Zalimun (polytheists and wrongdoers).) (52. Then after that We forgave you so that you might be grateful.) (53. And (remember) when We gave Musa the Scripture (the Tawrah)) and the criterion (of right and wrong) so that you may be guided aright.)