Acts 18:18-21. Paul leaves Corinth with Priscilla and Aquila and sails for Syria. In Cenchreae, "he cut his hair, for he had a vow." He then goes to Ephesus, where he enters the synagogue alone to argue with the Jews. They ask him to stay longer, but he declines, saying if God wills it, he will return. Then he sets sail from Ephesus.

A lot is happening in this short passage! Paul cut his hair because he had a vow. Nothing more is said of this vow, but the conjecture is the Paul made a Nazarite vow while in Corinth and cut his hair at the conclusion of that. Such a vow requires that the hair be burnt on the altar in Jerusalem, so this would mean Paul had decided to return to Jerusalem.

He then stays only a short time in Ephesus, even though the Jews ask him to stay longer. It seems like he is having success here, so why doesn't he stay longer? He says he will be back if it is God's will. It almost seems like his mind is already in Jerusalem. There will be trouble there, I'm sure Paul knows this, and perhaps he doesn't want to get too attached to the Ephesians or make promises he can't keep.

One might be tempted to question the prudence of making a vow at a time when it seems God needs Paul to be "all in" on evangelization, but Paul has shown that he is very much in touch with God's will for him. He would not have taken this vow if he didn't think God wanted him to. And perhaps the vow was what helped him in Corinth and what made the Jews in Ephesus so receptive to him. I could understand if the vision he had of God telling him to continue preaching in Corinth with God's protection prompted great thankfulness and a vow from Paul. Perhaps it was part of God's plan that the vision would inspire the vow, which would in turn help protect Paul and further his mission.