Musannaf Ibn Abi Shayba 19:966

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Hammad ibn Salamah, from Humayd, from Bakr: That a man accused another man of adultery (qadhf), so he brought him before Umar ibn al-Khattab. He (Umar) intended to flog him, but he (the accused) said, "I will produce the evidence." So he left him.