Narrated Zirr bin Hubaish:
I asked Ubai bin Ka'b regarding the two Muwwidhat (Surats of taking refuge with Allah). He said, "I asked the Prophet about them, He said, 'These two Surats have been recited to me and I have recited them (and are present in the Quran).' So, we say as Allah's Apostle ﷺ said (i.e., they are part of the Quran"
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 500
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 500