The Canons of the Council in Trullo; Often Called The Quinisext Council 53Canon LIII.

Whereas the spiritual relationship is greater than fleshly affinity; and since it has come to our knowledge that in some places certain persons who become sponsors to children in holy salvation-bearing baptism, afterwards contract matrimony with their mothers (being widows), we decree that for the future nothing of this sort is to be done. But if any, after the present canon, shall be observed to do this, they must, in the first place, desist from this unlawful marriage, and then be subjected to the penalties of fornicators.