142 Grk “heard they”; the referents (Paul and Silas) have been specified in the translation for clarity.
143 Roman citizens. This fact was disturbing to the officials because due process was a right for a Roman citizen, well established in Roman law. To flog a Roman citizen was considered an abomination. Such punishment was reserved for noncitizens.

In other versions

The police reported these words to the magistrates, and they were afraid when they heard that they were Roman citizens.

The officers reported this to the magistrates, and when they heard that Paul and Silas were Roman citizens, they were alarmed.

The police reported these words to the magistrates. They were afraid when they heard that Paul and Silas were Roman citizens.

When the police reported this, the city officials were alarmed to learn that Paul and Silas were Roman citizens.

So the police officers reported these words to the chief magistrates, and they were afraid when they heard that they were Roman citizens.

And the serjeants told these words unto the magistrates: and they feared, when they heard that they were Romans.

Public Domain