Is it permissible to bury something with the deceased in their grave, such as a prayer beads or an abaya, if the deceased had so willed?

It is not legislated to bury rosary beads or anything else with the deceased, as there is no mention of this in the Sharia or from the Salaf of the Ummah. Furthermore, the deceased's ownership of what he possessed ceases upon his death. A will concerning anything unlawful is not permissible, for every will involving disobedience or something forbidden is void. Nothing benefits a person in their grave except the good deeds they have presented, prayers for them, and the reward of righteous deeds dedicated to them.