Leviticus 5:1

When a man shall have sinned, and heard the voice of the oath of execration, or have been himself a witness, or shall have seen that cue of the world hath transgressed against the words of an oath, or shall have known that his companion hath sworn or imprecated vainly, if he show it not, he shall bear his sin.

anyone who sins, having heard admission of guilt specifically, regarding forbidden articles (as in, “you admitted your guilt” [Judges 17:2]). Although Scripture employs brevity, it implicitly warns the witness that he is obliged to testify, since if he does not testify, he is subject to Divine punishment: he has committed an offense . This applies when the witness forgot to testify, and afterward remembered.

AND HE IS A WITNESS, WHETHER HE HATH SEEN OR KNOWN. These are not three separate matters, for it is impossible that one be a witness without seeing and knowing. Rather, Scripture is stating that if one hear the voice of adjuration that a party to a law-suit adjures him, concerning a matter in which he is a witness either by seeing or knowing of it, if he does not tell it he has committed an iniquity. Now the witness is not obligated to bring this offering unless he knows such a testimony that the party in suit who adjured him [to give witness] would have legally won his case because of it [and the witness nonetheless withheld his evidence]. It is for this reason that our Rabbis interpreted [on the basis of the verse before us] that there is testimony which is valid by seeing without knowing, and [testimony which is valid] by knowing without seeing. How so? [Reuben says to Shimon:] “I have delivered to thee a maneh [as a loan] in the presence of such-and-such persons,” and [Shimon] claims “this never happened,). let the witnesses [you claim to have], come and testify.” This is a case of seeing without knowing [since although they saw Reuben handing the money to Shimon, they do not know the nature of this delivery, whether it was as a loan, or repayment of a loan that Shimon had originally made to him]. Reuben giving Shimon money is sufficiently valid to obligate Shimon to pay Reuben, although they did not know the nature of that transaction. If therefore they withhold their testimony, they are liable to bring the offering dealt with here. [If Reuben says to Shimon:] “You have admitted to owing me a maneh, in the presence of such-and-such persons,” [and Shimon replies]: “let them come and testify,” this is a case of knowing without seeing [and even though they did not see, they must testify what they know, and hence are liable if they withhold their evidence].
But in line with the plain meaning of Scripture, we need not [explain] the “seeing” here [to mean seeing] without knowing. Rather, the sense of the verse is as follows: whether he hath seen, meaning that he saw the loan or the sale completely [i.e., with knowledge], or known, i.e., that he heard [the defendants to the suit] admitting that transaction in the presence of witnesses, but he did not see it.
Now He does not state here: “and it is hidden from him,” [as He does in the following verse], because in this case [where the witness swears that he knows of no testimony for the party that adjures him to come and testify before the court], he is obligated to bring the offering [mentioned here in these verses] whether he swore [entirely] wilfully, or was in error on the oath whilst wilfully denying his knowledge of evidence. If, however, at the time of giving the oath he had forgotten the testimony, there is no guilt upon him.

ושמעה קול אלה, “by having heard an oath demanding him to testify, and having ignored it;” all the other paragraphs distinguish between inadvertently committed sins and those committed deliberately except the one involving an oath, known as שבועת בטוי, “a futile oath,” which is automatically considered as the one who uttered it doing so while aware of its implications. Solomon confirmed this in Kohelet 5,5 when he said: אל תתן את פיך לחטיא את בשרך ואל תאמר לפני המלאך כי שגגה היא, למה יקצוף האלוקים על קולך וחבל את מעשה ידיך, “do not let your mouth bring you in disfavour; and do not plead before the messenger (angel) that it was only an error; but fear the Lord else He may be angered by your talk and destroy your possessions.” When G–d will sit in judgment of all of His creatures, He will place such people next to the sorcerers and adulterers, as we know from the prophet Malachi (Malachi 3,5) והייתי עד ממהר במשפים ובמנאפים ובנשבעים (בשמי) לשקר, “I will act as relentless accuser against those that have no fear of Me, who practice sorcery, commit adultery and who swear falsely (using My name).”