Sunan Ibn Majah, Chapter: The Chapters on Divorce

from his father, from his grandfather, that the Prophet (saw) said: "If a woman claims that her husband has divorced her, and she brings a witness of good character (to testify) to that, her husband should be asked to swear an oath. If he swears, that will invalidate the testimony of the witness, but if he refuses then that will be equivalent to a second witness, and the divorce will take effect."