Musannaf Ibn Abi Shayba 19:117

Abu Bakr narrated to us, he said: Muhammad ibn Abd Allah al-Ansari narrated to us, from Ash'ath, from al-Hasan, he said: If four people testify to adultery against a man and they are not upright (just), the prescribed punishment (hadd) is averted from them, and none of them are flogged.