be a witness to something

Từ điển Oxford Advanced Learner 8th

be (a) ˈwitness to sth idiom
1. (formal)to see sth take place
• He has been witness to a terrible murder.
2. to show that sth is true; to provide evidence for sth
•His good health is a witness to the success of the treatment.
Main entry: ↑witness_idiom