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 (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." `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.