Matthew 14:3-4
3 Before that, Herod had commanded his soldiers, ‘Take hold of John. Tie his hands and feet and put him in prison.’ Herod had done this because of his wife Herodias. Before Herod married her, she was the wife of Herod's brother Philip. 4 John had said to Herod, ‘Herodias was your brother's wife. So it is not right for you to have her as your wife.’
14:4Herodias had left Philip, her husband. Herod married her. So John told him what Moses had written in the Law. The Law said, ‘A man must not marry his brother's wife while his brother is still alive.’ Herod was not happy with John when he told him that. So Herod sent his men to put John in prison.