Musannaf Ibn Abi Shayba 18:1158

Abu Bakr narrated to us, he said: Muhammad bin Bakr narrated to us, from Ibn Jurayj, he said: I said to 'Ata': "A man finds a man with his wife, so he kills him. Is his blood wasted (i.e., is there no retaliation)?" He said: "There is no matter except by evidence." I said: "If he testifies against him that he saw him with my family?" He said: "Even if he testifies, there is no matter except by evidence."