What is the ruling on sitting at or disturbing the grave of a non-Muslim, such as a Jew or a communist?

It is permissible to exhume the effaced grave of a polytheist if it is for a benefit, such as expanding a mosque or a road, due to the Prophet's (peace and blessings be upon him) statement: "He commanded the graves of the polytheists to be exhumed when he wanted to build the mosque." It is also permissible to exhume without need, because the polytheist is not sacred, whether alive or dead, and there is no harm in sitting on his grave for the same reason.