Musannaf Ibn Abi Shayba 19:421

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Mansur, from al-Harith, from Ibrahim, who said: Umar ibn al-Khattab said: "That I should suspend the prescribed punishments (hudud) due to doubts is more beloved to me than that I should establish them due to doubts."