Musannaf Ibn Abi Shayba 19:291

Abu Bakr narrated to us, he said: Abda narrated to us, from Sa’id, from Qatada, from al-Hasan, who said: If a man commits slander, the prescribed punishment (hadd) is established upon him. If he repeats the slander against him, there is no prescribed punishment upon him, unless he commits another act of slander.