Hadhrat Umar Radiallahu Anhu reports that Rasulullaah Sallallahu Alayhi Wasallam once said to him, "0 Umar! What would be your condition when you are in a piece of ground measuring four arm's lengths by two arm's lengths and when you see Munkar and Nakeer?"
"0 Rasulullaah Sallallahu Alayhi Wasallam" Hadhrat Umar Radiallahu Anhu asked, 'What is Munkar and Nakeer?"
Rasulullaah Sallallahu Alayhi Wasallam replied, 'They are the two examiners in the grave. They will dig the grave open with their canines and approach treading on their (extremely long) hairs. Their voices will be like devastating thunderclaps and their eyes like blinding lightning. They will both be carrying hammers so large that all the people of Mina are unable to even lift, yet for them it will be as if carrying this staff."
Rasulullaah Sallallahu Alayhi Wasallam then shook a little staff that he had in his hand. “They will then examine you. Should you fail to answer or delay, they will strike you with their hammers and reduce you to dust."
Hadhrat Umar Radiallahu Anhu then asked, "0 Rasulullaah! (Sallallahu Alayhi Wasallam) Will I be in the condition I am in now (in control of my senses)?"
When Rasulullaah Sallallahu Alayhi Wasallam replied in the affirmative, Hadhrat Umar Radiallahu Anhu said, "In that case, I shall be able to handle them."
Another narration adds that Rasulullaah Sallallahu Alayhi Wasallam then said to Hadhrat Umar Radiallahu Anhu, "I swear by the Being Who has sent me with the truth that Jibra'eel Alaihis Salaam has just informed me that when the two of them come to you and pose the questions, you will say to them, 'Allaah is my Rabb, who is yours? Muhammad Sallallahu Alayhi Wasallam is my Nabi, who is yours? Islaam is my Deen, what is yours?' They will then both exclaim, 'How strange! We do not know whether we have been sent to(question) you or whether you have been sent to (question) us!"' (1)
(1) Abdul Waahid Maqdasi in his book At-Tabseer, as quoted in Riyaadhun Nudhrah
Source: Hayaatus Sahaabah