Mark 6:17-20

For Herod himself had sent [guards] and had John arrested and shackled in prison because of Herodias, the wife of his [half-] brother Philip, because he (Herod) had married her. [Matt 14:3; Luke 3:19] For John had been saying to Herod, “It is not lawful [under Mosaic Law] for you to have your brother’s wife.” [Lev 18:16; 20:21] Herodias had a grudge against John and wanted to kill him, but she could not, because Herod feared John, knowing that he was a righteous and holy man, and he continually kept him safe. When he heard John [speak], he was very perplexed; but he enjoyed listening to him.