Sahih Bukhari Volume 9, Book 88, Hadith Number 216.

Narrated By Shaqiq : I heard Hudhaifa saying, “While we were sitting with ‘Umar, he said, ‘Who among you remembers the statement of the Prophet about the afflictions?’ Hudhaifa said, “The affliction of a man in his family, his property, his children and his neighbors are expiated by his prayers, Zakat (and alms) and enjoining good and forbidding evil.” ‘Umar said, “I do not ask you about these afflictions, but about those afflictions which will move like the waves of the sea.” Hudhaifa said, “Don’t worry about it, O chief of the believers, for there is a closed door between you and them.” ‘Umar said, “Will that door be broken or opened?” I said, “No. it will be broken.” ‘Umar said, “Then it will never be closed,” I said, “Yes.” We asked Hudhaifa, “Did ‘Umar know what that door meant?” He replied, “Yes, as I know that there will be night before tomorrow morning, that is because I narrated to him a true narration free from errors.” We dared not ask Hudhaifa as to whom the door represented so we ordered Masruq to ask him what does the door stand for? He replied, “‘Umar.”