Musnad Ahmad 804:1

Isma'il narrated to us, Al-Jurayri informed us, from Abu al-'Ala', who said: A man said: We were with the Messenger of Allah, may Allah bless him and grant him peace, on a journey, and the people were taking turns [riding], and there was a scarcity of mounts. The time for the Messenger of Allah, may Allah bless him and grant him peace, to dismount arrived, and my time to dismount arrived. He caught up with me from behind, struck my shoulder, and said: "Say: I seek refuge in the Lord of the daybreak." So I said: "I seek refuge in the Lord of the daybreak," and the Messenger of Allah, may Allah bless him and grant him peace, recited it, and I recited it with him. Then he said: "Say: I seek refuge in the Lord of mankind." So the Messenger of Allah, may Allah bless him and grant him peace, recited it, and I recited it with him. He said: "When you pray, recite them both."