Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: The Prophet Muhammad (peace and blessings be upon him) came to a secluded grave, and he performed the funeral prayer over it. We also performed the funeral prayer behind him. (Imam Ibn Hibban, may Allah have mercy on him, says:): In this narration, there is a clear statement: The Prophet Muhammad (peace and blessings be upon him) performed the funeral prayer over a grave that was in a corner. The word "manboodh" means "corner," so these words will guide you to the fact that performing the funeral prayer over a grave is permissible as long as it is new and in a corner that has not been dug up, or among graves that have not been dug up. As for those graves that are dug up and their soil is turned over, their soil becomes impure, and it is not permissible to pray on something impure. However, if a person stands on something clean and then performs the funeral prayer over a grave that has been dug up (then its ruling is different), and this ruling is other than that which is in a corner and has not been dug up.