Zirr bin Hubaysh (may Allah have mercy on him) said: I asked Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) about the Mu'awwidhatayn and said: O Abu al-Mundhir! Your brother, Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), does not count them as part of the Qur'an. He said: I asked the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "These words were said to me, so I said them." So we too say just as the Messenger of Allah (peace and blessings be upon him) said.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4045