It was his duty to give the evidence.
3 seconds sound clip from the The Third Man (1949) movie soundboard.
You can hear this line at 00:40:55.050 in the Blu-ray version of the movie.
Quote context
[...]
- Who could have told you a story like that?
- The porter at Harry's place. He was cleaning the window at the time.
- No, no, he didn't see the accident, but he saw three men carrying the body.
- Why wasn't he at the police inquiry?
- He didn't want to get involved.
- You'll never teach these Austrians to be good citizens.
- It was his duty to give the evidence.
- Even so, he remembers wrong.
- Harry was dead before you got him to that statue.
- He probably knows a lot more than that.
[...]