be a witness

See: observe, watch
References in classic literature ?
The beast is good, take him in what part you will, and it was to be food for man that he was fashioned; but I cannot say that I will be a witness and a helper to the waste of killing one daily.
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use without just compensation.
Irwine is to be a witness himself, by and by, on her side, you know, to speak to her character and bringing up.