Musnad Ahmad 482:9

Hafs ibn Ghiyath narrated to us, from Isma'il, from Qays, from Uqbah ibn Amir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Two surahs have been revealed to me, so seek refuge through them, for no one has sought refuge through anything like them." Meaning: The Mu'awwidhatayn (the two surahs of seeking refuge).