Kiddushin, Chapter 3, Mishnah 11. This mishnah is an expansion of the previous one, and it was brought here in passing, following on from it. In fact it contains no new information beyond what we already learned from the previous mishnah; it simply spells out that same principle in an additional case.
The claims in the mishnah:
The man says to the mother: "Kidashti es bitcha" - "I betrothed your daughter," "vehi omeres: lo kidashta ela osi" - and she says: "You betrothed no one but me" - that is, the mother denies that he betrothed her daughter and claims that he betrothed her herself. From these two conflicting claims, four rulings follow.
The four rulings that emerge from the mishnah:
"Shehu asur bikrovos ketanah" - the man is forbidden to the daughter's relatives, since by his own words he betrothed her.
"Uketanah muteres bikrovav" - the daughter is permitted to his relatives, since she herself said nothing one way or the other, and even had she made a claim, as a minor she is not believed.
"Hu mutar bikrovos gedolah" - the man is permitted to the mother's relatives, for although she claims that he betrothed her, he denies what she says.
"Ugedolah asurah bikrovav" - the mother is forbidden to his relatives, since by her own claim she is betrothed to him.
In summary: in this mishnah we learned the law of the contradictory claims between a man who says he betrothed the daughter and the mother who says he betrothed her: each side is forbidden to the relatives of the side to whom, by that side's own claim, the betrothal took place, while the opposing side, the one who denies it - or who has no standing to make a claim at all, like the minor - remains permitted.