Musnad Ahmad 928:2

Waki’ narrated to us, Sufyan narrated to us, from ‘Asim, from Zirr, who said: I asked Ubayy ibn Ka’b about the two *Mu’awwidhatayn* (the two surahs of seeking refuge). He said: I asked the Prophet, peace and blessings of Allah be upon him, about them, and he said: "It was said to me, so I said to you, so say [them]." Ubayy said: The Prophet, peace and blessings of Allah be upon him, said [that] to us, so we say [them].