Musnad Ahmad 30:1259

Abu al-Nadr narrated to us, Abu Aqil—his name is Abdullah ibn Aqil al-Thaqafi: he is trustworthy—narrated to us, Abdullah ibn Sa'id narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Your oath is that by which your companion believes you."