Musnad Ahmad 5:516

'Abdullah narrated to us, 'Uthman ibn Abi Shaybah narrated to us, 'Abdullah ibn Numayr narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Abu al-Bakhtari, from 'Ali ibn Abi Talib, who said: "When a hadith is narrated to you from the Messenger of Allah, may Allah bless him and grant him peace, then assume regarding it that which is most appropriate, that which is most guided, and that which is most pious."