Yosef Karo, Shulchan Aruch — Choshen Mishpat 37
“ A person is disqualified from any testimony in which he gains some kind of benefit by testifying. Therefore, if two people are partners in land and a third party attempts to remove the land from one of the partners by claiming that the individual who sold the land to them had originally stolen it from him, the other partner may not testify on behalf of his partner to disqualify the plaintiff’s witnesses. His testimony would not even help for his partner’s portion unless he removed himself from the field and they made a kinyan to give his portion to this partner. ”
