Musnad Ahmad 5:528

'Abdullah narrated to us, Abu Khaythama Zuhayr ibn Harb narrated to me, Jarir narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Abu al-Bakhtari, from Abu 'Abd al-Rahman al-Sulami, who said: 'Ali said: "When you are narrated a hadith 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."