Rashi on Leviticus 5:1/רש"י על ויקרא ה׳:א׳

1‎‏‎ קול האלה ‎ה‎ושמע‎ [AND IF A SOUL SIN] AND HEAR THE VOICE OF AN OATH in a matter to which he was witness, i. e. that he (the person interested in the evidence) called upon him (the witness) by an oath that if he knows any evidence favourable to him he should testify for him before the court (cf. Sifra); if he does not tell it, he bears his iniquity.