Sahih al-Bukhari 515 (Book 9, Hadith 4)

Narrated 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 Messenger (ﷺ) about the afflictions?’ I said, ‘I know it as the Prophet (ﷺ) 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 misstatement” The subnarrator
added that they deputized Masruq to ask Hudhaifa (about the door). Hudhaifa said, “The door
was `Umar himself.”