2,701. Only One Witness; Adultery

Hilchos Shevuos 10:5

If there’s only one witness, and he denies having information with an oath, he’s not liable for an oath of testimony because the testimony of one witness wouldn’t obligate the defendant to pay.

Hilchos Shevuos 10:6

Let’s say that the plaintiff summons two witnesses to testify that his wife committed adultery; they deny having information and take an oath. In such a case, the witnesses are liable for an oath of expression because if they testified, their testimony would have caused the woman to forfeit the value of her kesubah. In other words, the one who summoned them to testify would be exempted from paying it, rendering this a financial case.