What is the ruling on praying over an unknown deceased person, and is it permissible to pray over the ashes of his cremated body?

Prayer is obligatory upon the Muslim whose state is unknown. If the man is married to a non-scriptural disbeliever, his marriage is forbidden, and if he deems it permissible, he disbelieves after proof has been established against him. The default is the continuation of the ruling of Islam for someone whose Islam has been established with certainty. If his body is burned, prayers are offered over what remains of his ashes. If parts or ashes remain, prayers are offered over them after washing what is possible or performing tayammum.