Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī

The Prophet ﷺ strongly committed to fast the day of Ashura

bukhari:2006 – ʿUbaydullāh b. Mūsá > Ibn ʿUyaynah > ʿUbaydullāh b. Abū Yazīd > Ibn ʿAbbās

I never saw the Prophet ﷺ seeking to fast on a day that he favored more than another except this day, the day of 'Ashura', and this month, meaning the month of Ramadan.  

البخاري:٢٠٠٦ – حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى عَنِ ابْنِ عُيَيْنَةَ عَنْ عُبَيْدِ اللَّهِ بْنِ أَبِي يَزِيدَ عَنِ ابْنِ عَبَّاسٍ ؓ قَالَ

مَا رَأَيْتُ النَّبِيَّ ﷺ يَتَحَرَّى صِيَامَ يَوْمٍ فَضَّلَهُ عَلَى غَيْرِهِ إِلاَّ هَذَا الْيَوْمَ يَوْمَ عَاشُورَاءَ وَهَذَا الشَّهْرَ يَعْنِي شَهْرَ رَمَضَانَ  

See similar narrations below:

Collected by Nasāʾī, Nasāʾī's Kubrá
nasai:2370 – Qutaybah > Sufyān > ʿUbaydullāh > Ibn ʿAbbās And Suʾil > Ṣiyām ʿĀshūrāʾ

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.'  

النسائي:٢٣٧٠ – أَخْبَرَنَا قُتَيْبَةُ قَالَ حَدَّثَنَا سُفْيَانُ عَنْ عُبَيْدِ اللَّهِ أَنَّهُ سَمِعَ ابْنَ عَبَّاسٍ وَسُئِلَ عَنْ صِيَامِ عَاشُورَاءَ قَالَ

مَا عَلِمْتُ النَّبِيَّ ﷺ صَامَ يَوْمًا يَتَحَرَّى فَضْلَهُ عَلَى الأَيَّامِ إِلاَّ هَذَا الْيَوْمَ يَعْنِي شَهْرَ رَمَضَانَ وَيَوْمَ عَاشُورَاءَ  

nasai-kubra:2691 – Qutaybah b. Saʿīd > Sufyān > ʿUbaydullāh > Ibn ʿAbbās Yasʾal > Ṣiyām Yawm ʿĀshūrāʾ

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)  

الكبرى للنسائي:٢٦٩١ – أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ قَالَ حَدَّثَنَا سُفْيَانُ عَنْ عُبَيْدِ اللهِ سَمِعَ ابْنَ عَبَّاسٍ يَسْأَلُ عَنْ صِيَامِ يَوْمِ عَاشُورَاءَ قَالَ

«مَا عَلِمْتُ النَّبِيَّ ﷺ صَامَ يَوْمًا يَتَحَرَّى فَضْلَهُ عَلَى الْأَيَّامِ إِلَّا هَذَا الْيَوْمَ يَعْنِي شَهْرَ رَمَضَانَ وَيَوْمَ عَاشُورَاءَ»