Musannaf Ibn Abi Shayba 21:391

Abu Mu'awiyah narrated to us, from al-A'mash, from Jami' ibn Shaddad, from his father, who said: The first speech that Umar spoke was that he said: "O Allah, I am weak, so strengthen me; and I am harsh, so soften me; and I am stingy, so make me generous."