Sahih Muslim Book - 13, Hadith - 2662

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى شَيْبَةَ وَعَمْرٌو النَّاقِدُ جَمِيعًا عَنْ سُفْيَانَ - قَالَ أَبُو بَكْرٍ حَدَّثَنَا ابْنُ عُيَيْنَةَ - عَنْ عُبَيْدِ اللَّهِ بْنِ أَبِى يَزِيدَ سَمِعَ ابْنَ عَبَّاسٍ - رضى الله عنهما - وَسُئِلَ عَنْ صِيَامِ يَوْمِ عَاشُورَاءَ. فَقَالَ مَا عَلِمْتُ أَنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- صَامَ يَوْمًا يَطْلُبُ فَضْلَهُ عَلَى الأَيَّامِ إِلاَّ هَذَا الْيَوْمَ وَلاَ شَهْرًا إِلاَّ هَذَا الشَّهْرَ يَعْنِى رَمَضَانَ.

It was narrated from Ubaidullah bin Abi Yazid that he heard Ibn Abbas [may Allah be pleased with them] being asked about fasing on the day of Ashura. He said: “I do not know that the Messenger of Allah singled out any day for fasting, regarding it as superior to other days, apart from this day, or any month apart from this month” - meaning Ramadan.