Musnad Ahmad 482:64

Yazid ibn Harun narrated to us, Ismail ibn Abi Khalid informed us, from Qays ibn Abi Hazim, from Uqbah ibn Amir al-Juhani, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Verses have been revealed to me the likes of which I have never seen: the two refuge-seeking surahs (al-Mu'awwidhatayn)." Then he recited them.