Sahih Bukhari Volume 003, Book 031, Hadith Number 119.
Narated By 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.