Musnad Ahmad 27:696

'Abd al-Malik ibn 'Amr narrated to us, Nafi' ibn Abi Nu'aym narrated to us, from Nafi', from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "Indeed, Allah the Exalted has placed the truth upon the tongue of 'Umar and in his heart."