Mark 6:18
"For John had said unto Herod, It is not lawful for thee to have thy brother’s wife."
Cross References (ranked by relevance)
Leviticus 18:16
Thou shalt not uncover the nakedness of thy brother’s wife: it is thy brother’s nakedness.
Leviticus 20:21
And if a man shall take his brother’s wife, it is an unclean thing: he hath uncovered his brother’s nakedness; they shall be childless.
1 Kings 22:14
And Micaiah said, As the LORD liveth, what the LORD saith unto me, that will I speak.