Musannaf Ibn Abi Shayba 19:805

Abu Bakr narrated to us, he said: Sharik narrated to us, from Abdullah, from Ikrimah, who said: Umar said to Abd al-Rahman ibn Awf: "What do you think if you were the judge and the governor, and then you saw a person committing a crime punishable by a prescribed penalty (hadd), would you carry it out upon him?" He said: "No, not until someone else testifies with me." He said: "You have spoken correctly. And had you said other than this, I would not have permitted it."