The Oath is Rabbinic[, modeled after the Biblical oath of Partial Admission]: Bava Metzia 4a, 5b
Why the oath is required: Bava Metzia 5b-6a
Where there are witnesses, there is no Oath: Bava Metzia 2a
Dividing it
If one of them grabbed the object, before an oath could be sworn: Bava Metzia 6a
Above, where the other one was quiet at first and then protested: Bava Metzia 6a
Returning it
Whether one must swear that he didn't find more than he is returning: Bava Metzia 4b, 8a
