Musnad Ahmad 5:422

Yahya ibn Sa'id narrated to us, from Mis'ar, narrated to us 'Amr ibn Murrah, from Abu al-Bakhtari, from Abu 'Abd al-Rahman, 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, most rightly-guided, and most God-fearing."