Abd al-Rahman b. Yazid said:

When al-Ash'ath b. Qais entered the house of 'Abdullah he was having his breakfast. He ('Abdullah b. Umar) said: Abd Muhammad (al-Asha'th), come near to the breakfast. Thereupon he said: Is not today the day of 'Ashura? He ('Abd al-Rahman) said: Do you know what the day of 'Ashura is? He said: What is it? He said: It is a day on which the Messenger of Allah (ﷺ) used to observe fast before the (fasting) in the month of Ramadan (became) obligatory. But when it became obligatory the (fasting of 'Ashura) was abandoned (as compulsory). Abu Kuraib said: He (the Prophet) abandoned it.

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَأَبُو كُرَيْبٍ جَمِيعًا عَنْ أَبِي مُعَاوِيَةَ، قَالَ أَبُو بَكْرٍ حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ عُمَارَةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ يَزِيدَ، قَالَ دَخَلَ الأَشْعَثُ بْنُ قَيْسٍ عَلَى عَبْدِ اللَّهِ وَهُوَ يَتَغَدَّى فَقَالَ يَا أَبَا مُحَمَّدٍ ادْنُ إِلَى الْغَدَاءِ ‏.‏ فَقَالَ أَوَلَيْسَ الْيَوْمُ يَوْمَ عَاشُورَاءَ قَالَ وَهَلْ تَدْرِي مَا يَوْمُ عَاشُورَاءَ قَالَ وَمَا هُوَ قَالَ إِنَّمَا هُوَ يَوْمٌ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُهُ قَبْلَ أَنْ يَنْزِلَ شَهْرُ رَمَضَانَ فَلَمَّا نَزَلَ شَهْرُ رَمَضَانَ تُرِكَ ‏.‏ وَقَالَ أَبُو كُرَيْبٍ تَرَكَهُ ‏.‏