Musnad Ahmad 26:98

Yahya ibn Sa'id narrated to us, from Ibn 'Ajlan, who said: 'Awn narrated to me, from 'Abdullah ibn Mas'ud, who said: "When you are narrated a hadith from the Messenger of Allah, may Allah bless him and grant him peace, then assume regarding the Messenger of Allah, may Allah bless him and grant him peace, that which is most appropriate, most guided, and most pious."