Albert Barnes Commentary Acts 23:20

"And he said, The Jews have agreed to ask thee to bring down Paul tomorrow unto the council, as though thou wouldest inquire somewhat more exactly concerning him." — Acts 23:20 (ASV)

And he said, etc. In what way this young man had received information about this, we can only conjecture. It is likely that he was a student under one of the Jewish teachers, and that he might have learned it from him. It is highly unlikely that the purpose of the forty men would be kept very secret. Indeed, it is evident that they were not themselves very anxious about concealing their oath, as they mentioned it fully to the chief priests and elders (Acts 23:14).