Sahih Bukhari Volume 001, Book 010, Hadith Number 503.
Narated By Shaqiq : That he had heard Hudhaifa saying, "Once I was sitting with 'Umar and he said, 'Who amongst you remembers the statement of Allah's Apostle about the afflictions?' I said, 'I know it as the Prophet had said it.' 'Umar said, 'No doubt you are bold.' I said, 'The afflictions caused for a man by his wife, money, children and neighbor are expiated by his prayers, fasting, charity and by enjoining (what is good) and forbidding (what is evil).' 'Umar said, 'I did not mean that but I asked about that affliction which will spread like the waves of the sea.' I (Hudhaifa) said, 'O leader of the faithful believers! You need not be afraid of it as there is a closed door between you and it.' 'Umar asked, Will the door be broken or opened?' I replied, 'It will be broken.' 'Umar said, 'Then it will never be closed again.' I was asked whether 'Umar knew that door. I replied that he knew it as one knows that there will be night before the tomorrow morning. I narrated a Hadith that was free from any mis-statement" The subnarrator added that they deputed Masruq to ask Hudhaifa (about the door). Hudhaifa said, "The door was 'Umar himself."