Musannaf Ibn Abi Shayba 19:121

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from al-Shaybani, from al-Sha’bi, and from Hisham, from al-Hasan, that they both said: If he slanders a group all together, he is lashed one prescribed punishment. And if he slanders them separately, he is lashed for each one of them a prescribed punishment.