Hilchos Lashon Hara, Opening – negative commands (10)

If he stands up and testifies against him in beis din concerning some prohibition as a single witness, no benefit can result such as [obligating him to pay] money [or to swear] an oath, nor to disqualify this person from the assumption that he is kosher, since he is only one witness. Therefore, he is simply giving him a bad name and transgresses the negative command, “A single witness shall not stand [to give witness] about a man regarding any sin or any transgression.” The beis din is obligated to give him lashes for doing this.