Musnad Ahmad 482:59

Zayd ibn al-Hubab narrated to us, Mu'awiyah ibn Salih narrated to us, al-'Ala' ibn al-Harith narrated to us, from al-Qasim ibn 'Abd al-Rahman, the freed slave of Mu'awiyah ibn Abi Sufyan, from 'Uqbah ibn 'Amir, who said: I was leading the she-camel of the Messenger of Allah, may Allah bless him and grant him peace. He said to me: "Shall I not teach you two surahs, the likes of which have not been recited?" I said: "Yes." So he taught me: "Say, I seek refuge in the Lord of mankind" (Surah al-Nas) and "Say, I seek refuge in the Lord of the daybreak" (Surah al-Falaq). He did not see me impressed by them. When the morning came, he recited them, and then said to me: "How did you see them, O 'Uqbah?"