So when Paul's sister's son heard of their ambush, he went and entered the barracks and told Paul.
Acts 23:16
Surrounding context
23:15 Now you, therefore, together with the council, suggest to the commander that he be brought down to you tomorrow, as though you were going to make further inquiries concerning him; but we are ready to kill him before he comes near."
23:16 So when Paul's sister's son heard of their ambush, he went and entered the barracks and told Paul.
23:17 Then Paul called one of the centurions to him and said, "Take this young man to the commander, for he has something to tell him."