Musnad Ahmad 482:87

Waki' narrated to us, Ibn Abi Khalid narrated to us, from Qays, 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 have not been seen," or "the likes of which we have not seen," meaning the two *Mu'awwidhatayn* (Surah Al-Falaq and Surah An-Nas).