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

Narrated Abu Wail from Hudhaifa:

Umar asked the people, "Who remembers the narration of the Prophet about the affliction?" Hudhaifa said, "I heard the Prophet 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." 'Umar asked, "Will that gate be opened or broken?" He replied, "It will be broken." 'Umar said, "Then the gate will not be closed again till the Day of Resurrection." We said to Masruq, "Would you ask Hudhaifa whether 'Umar knew what that gate symbolized?" He asked him and he replied "He ('Umar) knew it as one knows that there will be night before tomorrow, morning.

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 ﷺ فِي الْفِتْنَةِ قَالَ حُذَيْفَةُ أَنَا سَمِعْتُهُ يَقُولُ " فِتْنَةُ الرَّجُلِ فِي أَهْلِهِ وَمَالِهِ وَجَارِهِ تُكَفِّرُهَا الصَّلاَةُ وَالصِّيَامُ وَالصَّدَقَةُ ". قَالَ لَيْسَ أَسْأَلُ عَنْ ذِهِ، إِنَّمَا أَسْأَلُ عَنِ الَّتِي تَمُوجُ كَمَا يَمُوجُ الْبَحْرُ. قَالَ وَإِنَّ دُونَ ذَلِكَ بَابًا مُغْلَقًا. قَالَ فَيُفْتَحُ أَوْ يُكْسَرُ قَالَ يُكْسَرُ. قَالَ ذَاكَ أَجْدَرُ أَنْ لاَ يُغْلَقَ إِلَى يَوْمِ الْقِيَامَةِ. فَقُلْنَا لِمَسْرُوقٍ سَلْهُ أَكَانَ عُمَرُ يَعْلَمُ مَنِ الْبَابُ فَسَأَلَهُ فَقَالَ نَعَمْ، كَمَا يَعْلَمُ أَنَّ دُونَ غَدٍ اللَّيْلَةَ.