Abu Qatadah reported: The Prophet, peace and blessings be upon him, was asked about fasting the day of Arafat. The Prophet said, “It will expiate the sins of the previous and upcoming years.” Then he was asked about fasting the day of Ashura. The Prophet said, “It will expiate the sins of the year.”
Source: Ṣaḥīḥ Muslim 1162
Grade: Sahih (authentic) according to Muslim
عَنْ أَبِي قَتَادَةَ عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ وَسُئِلَ عَنْ صَوْمِ يَوْمِ عَرَفَةَ فَقَالَ يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ وَالْبَاقِيَةَ قَالَ وَسُئِلَ عَنْ صَوْمِ يَوْمِ عَاشُورَاءَ فَقَالَ يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ
1162 صحيح مسلم كتاب الصيام باب استحباب صيام ثلاثة أيام من كل شهر وصوم يوم عرفة وعاشوراء والاثنين والخميس