witness to something
phrasal verb with witness verb [ T ] uk/ˈwɪt.nəs/ us/ˈwɪt.nəs/
UK formal
（尤指在法庭上）作证，证明…是真实的
to say publicly, especially in a law court, that something is true or that it happened
A handwriting expert witnessed to the authenticity of the letter. 笔迹专家证明了这封信不是伪造的。
[ + -ing verb ] She witnessed to having seen the robbery take place. 她作证说目睹了抢劫的过程。