أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عُبَيْدِ اللَّهِ، أَنَّهُ سَمِعَ ابْنَ عَبَّاسٍ، وَسُئِلَ، عَنْ صِيَامِ، عَاشُورَاءَ قَالَ مَا عَلِمْتُ النَّبِيَّ صلى الله عليه وسلم صَامَ يَوْمًا يَتَحَرَّى فَضْلَهُ عَلَى الأَيَّامِ إِلاَّ هَذَا الْيَوْمَ يَعْنِي شَهْرَ رَمَضَانَ وَيَوْمَ عَاشُورَاءَ ‏.‏

It was narrated from 'Ubaidullah that: 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.' Translation Not Available

Book Ref: Sunan An Nisai Book 22 Hadith 2370
Web Ref:  Sunan An Nisai Vol 3 Book 22 Hadith 2372

---