Leviticus 5:1 And if a person sins and hears the demand for an oath, and he is [a] witness – has either seen or known of it – so that if he does not speak out, he shall bear his sin.
Gemora Baba Kama 56a: He shall bear his sin – vis-à-vis heavenly justice [he is guilty] but in terms of human courts he is not liable.
Torah Temimah Colloquial Translation on Note #27:
The reason why he is not liable in human courts is because [the witness who does not testify] is not directly causing the loss to the victim [by his not testifying]. The Gemora’s conclusion is that even a single witness is indeed still obligated to come forward and testify about what he knows. The commentaries explain that even though a single witness who does not come forward is exempt from the punishment of bringing a sacrifice [korban shavua] since, by himself, his testimony would not have been enough to convict the perpetrator; nevertheless he is still obligated to come forward and give testimony anyway. This is because his single testimony would be enough to cause the suspected perpetrator to have to undergo a biblical oath to protest his innocence. Sometimes, the suspected perpetrator would prefer to pay [the amount that the single witness claims is owed] rather than undergo such an oath. Therefore, we see that even a single witness could provide relief to the victim, even though failure to testify would not make him culpable for any punishment.
Nevertheless, this issue is still difficult to understand. The reason why this explanation above is difficult to understand is because the Gemora understands this posuk to refer to a case of two witnesses. This would include the whole posuk, even the second part that says “and he shall bear his sin”. (This is explained in the previous commentary that the whole posuk is dealing with a case of two witnesses.) If this is the case, then how could one possibly deduce from some type of forced logic that a single witness also has to testify since the perpetrator might not want to swear, etc. The commentaries have, in fact, toiled hard to explain this. See the notes of the Choshen Mishpat, Section 28.
[Consequently], it seems to me that the true obligation of a single witness to testify is not because of the section in our posuk where it says “he will bear his sin”. Rather the source of the requirement for a single witness to testify is actually from a different lesson in Torat Cohanim on Parshat Kedoshim on the posuk “do not stand idly by while your brother’s blood is spilt”. There the Torat Cohanim comments “from where do we know that if you have knowledge, that it is not appropriate for you to keep silent? [The source] is from this posuk: don’t stand idly by while your brother’s blood is spilt”.
The issue is that everyone is obligated to save his friend from any trouble that he may be in. This applies whether the trouble is physical danger or monetary trouble. Therefore, since even a single witness can bring some [relief] to the victim. As we explained above this would be through requiring the suspected perpetrator to take a biblical oath, and perhaps the person would rather pay than swear. It is therefore clear that even a single witness has some ability to assist a victim. Due to this fact, he is, [of course] required to testify.
Additionally, it is clear in my eyes that even though the Gemora doesn’t mention this posuk in Parshat Kedoshim, nevertheless I am practically positive that the main source for the single witness to testify is from the posuk in Parshat Kedoshim [about not standing idly by] and from the lesson learned from that posuk by Torat Cohanim. We already know about similar lessons derived from this same posuk in Parshat Kedoshim. For example, in Gemora Sanhedrin 73a where it asks “from where do we know that a person who sees his friend drowning in the sea or he sees that thieves are about to come upon his friend, that one is obligated to save him? The answer is: Don’t stand idly by while your brother’s blood is spilt.”
[Keep in mind] that the word “blood” also can mean “money”. This is how we know that our posuk of not standing idly by is also the source of the law requiring one to help if one sees someone attempting to steal from one’s friend through false statements.
Editor’s Note: The Torah Temimah takes this opportunity to bring a lesson from another parsha to remind us that the commandment to “not stand idly by” applies to every possible difficulty and issue in which you can perhaps assist your friend. You are guilty in the eyes of heaven if you could have assisted and you didn’t.