Musnad Ahmad 482:101

'Abd al-Rahman ibn Mahdi narrated to us, from Mu'awiyah—meaning Ibn Salih—from al-'Ala' ibn al-Harith, from al-Qasim, the freed slave of Mu'awiyah, from 'Uqbah ibn 'Amir, who said: I was leading the riding beast of the Messenger of Allah, may Allah bless him and grant him peace, during a journey. He said: "O 'Uqbah, shall I not teach you the two best surahs that have been recited?" I said: "Yes." He said: "Say: 'I seek refuge in the Lord of the daybreak' (al-Falaq), and say: 'I seek refuge in the Lord of mankind' (an-Nas)." When he descended, he led the morning prayer with them. He then said: "How do you see them, O 'Uqbah?"