Mark 6:17
"For Herod himself had sent forth and laid hold upon John, and bound him in prison for Herodias’ sake, his brother Philip’s wife: for he had married her."
Cross References (ranked by relevance)
Matthew 11:2
Now when John had heard in the prison the works of Christ, he sent two of his disciples,
Matthew 4:12
Now when Jesus had heard that John was cast into prison, he departed into Galilee;
Luke 3:1
Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judaea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of Ituraea and of the region of Trachonitis, and Lysanias the tetrarch of Abilene,