Kesubos, chapter 9, Mishnah 7. This Mishnah lists several cases in which a woman may collect her kesubah only by taking an oath - an oath in which she declares before her husband or before the court that she has not yet collected any of her kesubah money.
"HaPogemes kesubasah":
A woman who comes to collect her kesubah and admits that she has already collected part of it, while her husband claims that she has already collected the entire kesubah. In this case - "lo tipara ela bishvuah" - she may not collect the balance she is claiming unless she swears that she has not collected more than she has admitted.
The Gemara explains the reason: a person paying off a debt is not careful to keep track of the amounts he has paid, whereas the one receiving payment is careful to remember what he has received. There is therefore a concern that she received more than she admits, and so an oath is imposed upon her in order to set her husband's mind at ease. Since we assume that her memory in this matter is more precise, we ask her to swear.
"Ed echad meid shehi peruah":
The husband claims that the kesubah has already been paid, the woman responds that she has not yet collected it, and the husband has one witness who supports his claim. Two witnesses are required in order to force collection, and a single witness is not enough. She may therefore collect, but only if she swears - in order to set her husband's mind at ease, since he maintains that she has already collected.
It is worth noting that this law does not follow the pattern of Torah law. By Torah law, when a single witness testifies that a person owes money, the defendant swears in order to exempt himself from paying - that is, the oath comes to protect the position of the one who denies the debt. Here, by contrast, the witness testifies that the woman has already received her payment and has no right to collect, and the oath serves specifically to enable her to collect. This, then, is not a Torah oath but a rabbinic enactment.
Additional cases in which she collects only by taking an oath:
"Minichsei yesomim" - a woman who comes to collect from the estate that is now in the hands of the orphans.
"Minechasim meshubadim" - a woman who comes to collect from estate property that has been sold, even if the husband sold it during his lifetime, since her claim by virtue of the kesubah takes precedence with regard to that property.
"Shelo befanav" - a woman who comes to collect her kesubah in her husband's absence, and turns to the court to pay her the kesubah when he is not present.
In each of these cases - "lo tipara ela bishvuah" - she is obligated to swear before she collects.
In summary: This Mishnah listed the cases in which a woman collects her kesubah only by taking an oath: the woman who admits she has collected part of her kesubah, out of concern that she received more than she admitted; the case of one witness testifying that the kesubah was paid, where her oath enables her to collect and is a rabbinic enactment rather than a Torah law; and the woman collecting from the property of orphans, from mortgaged property, or in her husband's absence.