Abu Qatadah reported: The Prophet, peace and blessings be upon him, was asked about fasting the day of ‘Arafat, and he said, “It will expiate the sins of the previous and upcoming years.” Then, the Prophet was asked about fasting the day of ‘Ashura, and he said, “It will expiate the sins of the past year.”
Source: Ṣaḥīḥ Muslim 1162
Grade: Sahih (authentic) according to Muslim
عَنْ أَبِي قَتَادَةَ عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ سُئِلَ عَنْ صَوْمِ يَوْمِ عَرَفَةَ فَقَالَ يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ وَالْبَاقِيَةَ 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
