Musannaf Ibn Abi Shayba 19:883

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from 'Abd al-Malik, he said: I heard al-Sha'bi say regarding a man who says to a man, "O adulterer," while he knows that he has committed adultery: "Is he to be lashed?" He said: "Yes, for Allah says: '...and then they do not bring four witnesses.'"