ANSWER:

This is not part of assignments that people would be rewarded for as after death, there is no accountability. 

This is a form of enjoyment the messengers of Allah are blessed with in their graves where such prayers is part of their favors and blessings from Allah that they enjoy. 

QUESTION: There is a hadith about the Prophet saying: “I passed by the grave of Musa, peace be upon him, and he was praying in his grave.”. If yes is it true that dead people can pray and earn good deeds because we know the deeds of a person gets cut off after death except for three things as mentioned in another hadith.