From Sahih Bukhari (Hadith 4977):
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 Qur'an)." Ubai said, "I asked Allah's Messenger (ﷺ) about them, and he said, 'They have been revealed to me, and I have recited them (as a part of the Qur'an)," So Ubai added, "So we say as Allah's Messenger (ﷺ) has said
Reference: Bukhari 4977