Musnad Ahmad 1034:171

Yahya ibn Sa’id narrated to us, from al-A’mash, Shaqiq narrated to me, he said: I heard Hudhayfah. And Waki’ narrated from al-A’mash, from Shaqiq, from Hudhayfah. And Muhammad ibn ‘Ubayd narrated to us, and he said: I heard Hudhayfah say: We were sitting with ‘Umar, and he said: "Which of you remembers the saying of the Messenger of Allah, may Allah bless him and grant him peace, regarding the trial (fitnah)?" I said: "I, just as he said it." He said: "You are indeed bold regarding it—or regarding him." I said: "The trial of a man in his family, his wealth, his children, and his neighbor is expiated by prayer, charity, enjoining good, and forbidding evil." He said: "That is not what I intend. Rather, the trial that surges like the surging of the sea." I said: "There is no harm upon you from it, O Commander of the Faithful, for between you and it is a closed door." He said: "Will it be broken or opened?" I said: "Rather, it will be broken." He said: "Then it will never be closed." We said: "Did ‘Umar know who the door was?" He said: "Yes, just as he knows that before tomorrow there is a night." Waki’ said in his narration: Masruq said to Hudhayfah: "O Abu ‘Abd Allah, did ‘Umar know what you narrated to him?" We said: "Did ‘Umar know who the door was?" He said: "Yes, just as he knows that before tomorrow there is a night. I narrated to him a narration that is not erroneous." We were in awe of Hudhayfah to ask him who the door was, so we ordered Masruq, and he asked him. He said: "The door is ‘Umar."