Nehemiah 13:29

That covenant made between me and his progenitors for themselves and their posterity, whereby I promised to give them an everlasting priesthood, Numbers 25:12,13, and they covenanted with me that they would faithfully and holily execute that sacred function according to the rules which I gave them, whereby, among other things, they were enjoined to keep themselves pure from all unlawful marriages, and from all other things which might pollute them or the priesthood.