Prophet Muhammad (Salllahu alayhi wasalaam): When a dead person is buried two black-blue angels come to him. One of them is called Munkar and the other Nakeer. (Tirmidhi)

And know that when death approaches you it will be instant & least expected just like when Munkar & Nakeer, the 2 Angels, whose job will be to question you in your grave will be at your appointed time.

... And the two Angels will come to him, sit him up and ask him, "Who is your Lord?" He will say, "O, oh! I don't know." They will ask him, "What is your religion?" and he will say "O, oh! I don't know." They will ask him, "What do you say about this man (Prophet Muhammad) who was sent to you?" He will say, "O, oh! I don't know!