All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Concerning the description of the two angels responsible for questioning the dead in the grave, it was narrated on the authority of Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam, said:
Once the dead are put in their graves (or he said: one of you), two black and blue angels will come to him; one named Al-Munkar and the other An-Nakeer. They will say, "What did you say about this man (the Prophet, sallallaahu ‘alayhi wa sallam)?" He will answer that he used to say, "He is the Slave and Messenger of Allaah. I testify that there is no deity worthy of worship but Allaah and that Muhammad is His Slave and Messenger." They will then say, "We knew that you were saying this." Then his grave will be widened by seventy cubits by seventy, lit for him, and it will be said to him, "Sleep". He will say, "May I return to my family to tell them?" They will say, "Sleep like a bridegroom who is not awakened except by his most beloved ones," and he will sleep until Allaah resurrects him from this bed of his. However, if he is a hypocrite, he will say, "I heard the people saying this and so I said like them. I do not know". They will thus say, "We knew you were saying this." It is then said to the earth, "Be welded on him", and so it will be welded on him and his ribs crush together and he remains there tormented until Allaah resurrects him from this bed of his.
Shaykh Al-Albaani said in As-Silsilah As-Saheehah that this narration is Saheeh (authentic).
Allaah Knows best.