Summary

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.
Source: Wikisource

Yosef Karo Shulchan Aruch — Choshen Mishpat 28

Paragraph 21- The acceptance of testimony requires three experts who are familiar with the rules of valid and invalid witnesses and are careful that each testimony conforms with the other, as the Mishna says, “one must be careful with his words lest he learn to lie from them.” Anyone who accepts testimony and is not fit to judge, is as if he accepted false testimony.
Source: Wikisource

Yosef Karo Shulchan Aruch — Choshen Mishpat 28

A person is prohibited from testifying regarding a matter he does not know, even if it was told to him by a truthful person. Even if one were to tell him to merely stand with another witness, but not actually testify, in order to intimidate the borrower who will think he has two witnesses and get the borrower to confess, the witness may not do so.
Paragraph 2- A person is permitted to place a cherem in the synagogue on anyone who knows testimony for him to come and testify. He cannot, however, force them to swear. If they don’t testify, they will carry their sin.
Source: Wikisource

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