he heard Ibn 'Abbas, when he was asked about the fast of 'Ashura' say: "I do not know that the Prophet fasted any day because of its virtue, except this day" meaning the month of Ramadan and the day of Ashura.' (Using translation from Nasāʾī 2370)
«مَا عَلِمْتُ النَّبِيَّ ﷺ صَامَ يَوْمًا يَتَحَرَّى فَضْلَهُ عَلَى الْأَيَّامِ إِلَّا هَذَا الْيَوْمَ يَعْنِي شَهْرَ رَمَضَانَ وَيَوْمَ عَاشُورَاءَ»