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 (ﷺ) about them, He said, 'These two Surats have been recited to me and I have recited them (and are present in the Qur'an).' So, we say as Allah's Messenger (ﷺ) said (i.e., they are part of the Qur'an."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا سُفْيَانُ، عَنْ عَاصِمٍ، وَعَبْدَةَ، عَنْ زِرِّ بْنِ حُبَيْشٍ، قَالَ سَأَلْتُ أُبَىَّ بْنَ كَعْبٍ عَنِ الْمُعَوِّذَتَيْنِ، فَقَالَ سَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ قِيلَ لِي فَقُلْتُ فَنَحْنُ نَقُولُ كَمَا قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم‏.‏
Reference : Sahih al-Bukhari 4976
In-book reference : Book 65, Hadith 498
USC-MSA web (English) reference : Vol. 6, Book 60, Hadith 500
  (deprecated numbering scheme)