Zirr bin Hubaysh (may Allah have mercy on him) said: I asked Ubayy bin Ka'b (may Allah be pleased with him) about the Mu'awwidhatayn, and he said: I asked the Messenger of Allah (peace and blessings be upon him) about the Mu'awwidhatayn, and he (peace and blessings be upon him) said: "It was said to me, so I said it."
"So we say just as the Messenger of Allah (peace and blessings be upon him) said."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4044