Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) was delivering a sermon when a man from Banu Layth bin Bakr came. Then he mentioned the long hadith of his confession and the woman's denial. He was unmarried, so he (peace and blessings be upon him) gave him a hundred lashes and then said: "Bring witnesses against the one you have accused." He had no witnesses, so he (peace and blessings be upon him) applied the prescribed punishment for slander to him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17134