John Gill Commentary Acts 22:26

"And when the centurion heard it, he went to the chief captain and told him, saying, What art thou about to do? for this man is a Roman." — Acts 22:26 (ASV)

When the centurion heard that
The question put by Paul, which strongly suggested that he was a Roman:

he went and told the chief captain; what Paul had said: saying, take
heed what you do;
or "are about to do"; lest some bad consequences should follow; lest he should affront the Roman people and senate, and lose his place, if not incur some corporeal punishment:

for this man is a Roman;
and it can never be answered to bind and beat a Roman.