Musannaf Ibn Abi Shayba 18:1156

Abu Bakr narrated to us, he said: Asbat narrated to us, from al-Shaybani, from Salamah, who said: A man was brought to Mus'ab who had found a man with his wife and killed him, so he declared his blood to be of no consequence (i.e., he did not punish the killer).