Charles Ellicott Commentary Matthew 14:3

"For Herod had laid hold on John, and bound him, and put him in prison for the sake of Herodias, his brother Philip`s wife." — Matthew 14:3 (ASV)

Put him in prison — Josephus (Antiquities, Book 18, Chapter 5, Section 2) gives Machaerus, in Perea, as the scene of the imprisonment and death of the Baptist.