Yosef Karo, Shulchan Aruch — Choshen Mishpat 28
“ Paragraph 1- Anyone who knows testimony that another has a use for, and is fit to testify, is obligated to testify if he is called to testify in court. This is true whether he is solo or has another witness with him. If he withholds his testimony, he is exempt from judgement by man but is liable by the laws of the heavens. A solo witness should only testify on monetary matters where he may require an oath or for a matter of prohibition where he is causing someone to separate from the prohibition. ”
