Musnad Ahmad 1085:3

Ali ibn Bahr narrated to us, Isa ibn Yunus narrated to us, al-Awza’i narrated to us, from Abdullah ibn Sa’d, from al-Sanabih, from Mu’awiyah, from the Prophet, peace and blessings of Allah be upon him, that he: "Forbade the *ghulutat* (the difficult, intricate, or controversial questions)."