Abu Qatadah al-Ansari (radiyallahu ‘anhu) said regarding the Prophet ﷺ:

سُئِلَ عَنْ صَوْمِ يَوْمِ عَرَفَةَ فَقَالَ: يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ وَالْبَاقِيَةَ وَسُئِلَ عَنْ صَوْمِ يَوْمِ عَاشُورَاءَ فَقَالَ: يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ

He was asked about fasting on the Day of ʿArafah and said, “It expiates the sins of the previous and following years” and he was asked about fasting of the Day of ʿAshuraʾ and said, “It expiates the sins of the previous year.” (Sahih Muslim [no. 1162])

Shaykh al-Islam Ibn Taymiyyah (rahimahullah) said:

It has been authentically reported from the Prophet ﷺ that he said fasting on the Day of ʿArafah expiates the sins of two years whilst fasting the Day of ʿAshuraʾ expiates the sins of one year. However, the unrestricted statement that it—‘expiates’—does not necessitate the understanding that it expiates the major sins without repentance for he ﷺ also said regarding one Jumu‘ah to another Jumu‘ah and one Ramadan to another Ramadan that they expiate the sins committed between them as long as one avoids the major sins (Sahih Muslim). It is known that the prayer is better than fasting and fasting the month of Ramadan is greater than fasting the day of ʿArafah; these do not expiate sins unless one avoids the major ones as specified by the Prophet ﷺ. As a result, how can it be thought that fasting one or two voluntarily days will expiate the sins of illicit sexual relations, stealing, drinking alcohol, gambling, magic and the like? This is not the case.”

Source: Mukhtasar al-Fatawa al-Misriyyah (p. 290) of Badr al-Din Muhammad ibn ʿAli al-Baʿli