Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 501
Narrated Zirr bin Hubaish:
I asked Ubai bin Ka'b, "O Abu AlMundhir! Your brother, Ibn Mas'ud said so-and-so (i.e., the two Mu'awwidh-at do not belong to the Quran)." Ubai said, "I asked Allah's Apostle about them, and he said, 'They have been revealed to me, and I have recited them (as a part of the Quran)," So Ubai added, "So we say as Allah's Apostle has said."