Kapłańska 5:1 Halacha: Mishneh Torah, Testimony, Shulchan Arukh, Choshen Mishpat, Mishneh Torah, Positive Mitzvot & Sefer HaChinukh

The witness is commanded to testify in religious court giving all testimony which he knows, whether his testimony will cost his friend or benefit him. This is when he [the witness] is selected to testify in a monetary case, as it is said (Leviticus 5:1) "If he is a witness or saw or knew, if he does not tell, he shall bear his sin."

[If] one of the litigants pleaded, 'There are witnesses or [other] evidence in my favour but I know not through whom this [could be made available],'¹⁵Tur citing Asheri in Resp. the Judge is obliged to pronounce the ban against any person who knows [aught] in his favour through witnesses or [other] evidence, [viz.,] that he make this known to the Judge,¹⁶ B.K. 56a where among the acts which are enumerated by R. Joshua for which an offender is exempt from the judgments of man but held responsible to the judgments of Heaven, is found the following: ‘One who knows evidence in favour of his fellow but does not testify on his behalf. What is the case before us? If a case where there are two (witnesses), is it not self-evident that it is based on a Biblical verse (why then should it be mentioned here?), (for it is written), If he do not utter it then he shall bear his iniquity (Lev. V, 1)? — It refers to a case where there is one (witness whose testimony merely imposes an oath upon the defendant — v. Sheb. 40a).’ We thus see that two witnesses who withhold evidence commit a Scriptural offence and even in the case of one witness, he is still held responsible to the judgments of Heaven. Consequently, we should do everything in our power to restrain one from transgression. , infra n. 21. even if the one who is his opponent [in Court] knows about the favourable opinion of the witnesses and the [other] evidence, he is obliged to state [it].¹⁷B.K. ibid.: ‘To hire false witnesses to give evidence (is an offence for which the offender is exempt from the judgments of Man, but is held responsible to the judgments of Heaven). What are the circumstances? If for his own benefit (i.e., in order to exact money illegally) should he not be required to pay the money (which he obtained through the false witnesses) and would he not be held responsible even to the judgments of Man? — It means for the benefit of his neighbour.’ Hence, we see that where he knows that his fellow is right, he would be liable to return the illegitimately acquired object. Cf. Tosaf. a.l. s.v. ממונא. , infra § 28, 2, Gloss. Gloss: , infra § 71, 7-8 in what manner we pronounce the ban and v. infra § 92, 6 [the law] if [the litigant] pleads that his opponent is ineligible to take an oath whether we pronounce the ban concerning this.

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. Nevertheless, if the court sees a need to have them swear to say the truth, they may do so. See earlier 17:3 and later 71:7-8. There are those that say that when a cherem is placed, even relatives and the party himself must testify. There are those that disagree and this is the primary ruling. See Yoreh Deah Siman 232 regarding a king who commands the placement of a cherem for testimony.