Musannaf Ibn Abi Shayba 19:963

Abu Bakr narrated to us, he said: Ibn al-Mubarak narrated to us, from Uthman ibn al-Aswad, who said: Ata was asked about a man who hears a man accusing another man [of adultery/slander]: "Should he inform him?" He said: "No. You only sit together by trust."