daf bit: Ketubot 87

If a woman is due her ketubah and says she has already received part of it (this is referred to as "impairing" her ketubah), she is not paid the balance unless she first takes an oath affirming her claim. If one witness testifies that she has been paid in full, she is not paid unless she takes an oath (but if she takes the oath she is paid, because there are not two witnesses). She cannot take payment from the property of orphans (that is, his children), from mortgaged property, or from an absent husband (one who has gone across the sea and not returned) unless she takes an oath. R. Shimon ruled that whenever she seeks to claim her ketubah the heirs may impose an oath on her. (87a)

Oaths are a serious matter because an oath carries the possibility of a false oath, which is effectively false testimony before God. So even if you're speaking truthfully, you don't do it if you don't have to and the halacha doesn't impose it lightly. (This is why, today, some Jews who have no intention of doing otherwise will still not agree to "swear to tell the truth" etc in courts. When I've been seated on juries I've "affirm"ed my service instead.)

That said, these kinds of requirements that people take oaths to settle claims come up in other cases of torts too; the ketubah case is not especially unusual.