Musnad Ahmad 547:3

Harun narrated to us, Ibn Wahb narrated to us, al-Layth narrated to me, from Hunayn ibn Abi Hakim, who narrated to him from Ali ibn Rabah al-Lakhmi, from Uqbah ibn Amir al-Juhani, who said: "The Messenger of Allah, may Allah bless him and grant him peace, commanded me to recite the *Mu'awwidhat* (the verses of seeking refuge) at the end of every prayer."