As-Saum (the fasting) is an expiation (for sins)

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، حَدَّثَنَا جَامِعٌ، عَنْ أَبِي وَائِلٍ، عَنْ حُذَيْفَةَ، قَالَ قَالَ عُمَرُ ـ رضى الله عنه ـ مَنْ يَحْفَظُ حَدِيثًا عَنِ النَّبِيِّ صلى الله عليه وسلم فِي الْفِتْنَةِ قَالَ حُذَيْفَةُ أَنَا سَمِعْتُهُ يَقُولُ ‏ "‏ فِتْنَةُ الرَّجُلِ فِي أَهْلِهِ وَمَالِهِ وَجَارِهِ تُكَفِّرُهَا الصَّلاَةُ وَالصِّيَامُ وَالصَّدَقَةُ Narrated Abu Wail from Hudhaifa: Narrated Abu Wail from Hudhaifa: Narrated Abu Wail from Hudhaifa: `Umar asked the people, "Who remembers the narration of the Prophet (ﷺ) about the affliction?" Hudhaifa said, "I heard the Prophet (ﷺ) saying, 'The affliction of a person in his property, family and neighbors is expiated by his prayers, fasting, and giving in charity." `Umar said, "I do not ask about that, but I ask about those afflictions which will spread like the waves of the sea." Hudhaifa replied, "There is a closed gate in front of those afflictions." : Sahih al-Bukhari 1895