Musnad Ahmad 5:421

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from 'Amr ibn Murrah, from Abu al-Bakhtari, from 'Ali, 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 guided, that which is most appropriate, and that which is most pious."