Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 500
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 said (i.e., they are part of the Quran"