Sahih Bukhari, Chapter: Obligatory Charity Tax (Zakat)

Hudhaifa said, "Umar said, 'Who amongst you remembers the statement of Allah's Apostle (p.b.u.h) about afflictions?' I said, 'I know it as the Prophet had said it.' Umar said, 'No doubt, you are bold. How did he say it?' I said, 'A man's afflictions (wrong deeds) concerning his wife, children and neighbors are expiated by (his) prayers, charity, and enjoining good.' (The sub-narrator Sulaiman added that he said, 'The prayer, charity, enjoining good and forbidding evil.') 'Umar said, 'I did not mean that, but I ask about that affliction which will spread like the waves of the sea.' I said, 'O chief of the believers! You need not be afraid of it as there is a closed door between you and it.' He asked, 'Will the door be broken or opened?' I replied, 'No, it will be broken.' He said, 'Then, if it is broken, it will never be closed again?' I replied, 'Yes.' " Then we were afraid to ask what that door was, so we asked Masruq to inquire, and he asked Hudhaifa regarding it. Hudhaifa said, "The door was Umar. "We further asked Hudhaifa whether Umar knew what that door meant. Hudhaifa replied in the affirmative and added, "He knew it as one knows that there will be a night before the tomorrow morning."