Acts 12:1-10 NIV
It was about this time that King Herod arrested some who belonged to the church, intending to persecute them. He had James, the brother of John, put to death with the sword. When he saw that this met with approval among the Jews, he proceeded to seize Peter also. This happened during the Festival of Unleavened Bread. After arresting him, he put him in prison, handing him over to be guarded by four squads of four soldiers each. Herod intended to bring him out for public trial after the Passover.
So Peter was kept in prison, but the church was earnestly praying to God for him.
The night before Herod was to bring him to trial, Peter was sleeping between two soldiers, bound with two chains, and sentries stood guard at the entrance. Suddenly an angel of the Lord appeared and a light shone in the cell. He struck Peter on the side and woke him up. “Quick, get up!” he said, and the chains fell off Peter’s wrists.
Then the angel said to him, “Put on your clothes and sandals.” And Peter did so. “Wrap your cloak around you and follow me,” the angel told him. Peter followed him out of the prison, but he had no idea that what the angel was doing was really happening; he thought he was seeing a vision. They passed the first and second guards and came to the iron gate leading to the city. It opened for them by itself, and they went through it. When they had walked the length of one street, suddenly the angel left him.
NIV: New International Version