كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(46)
Chapter: The virtue of reciting al-Mu`awwidhatain (the two surahs seeking refuge with Allah)
(46)
باب فَضْلِ قِرَاءَةِ الْمُعَوِّذَتَيْنِ ‏
Sahih Muslim 814 a

'Uqba b. 'Amir reported Allah's Messenger (ﷺ) as saying:

What wonderful verses have been sent down today. the like of which has never been seen! They are:" Say: I seek refuge with the Lord of the dawn," and" Say: I seek refuge with the Lord of men."
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا جَرِيرٌ، عَنْ بَيَانٍ، عَنْ قَيْسِ بْنِ أَبِي حَازِمٍ، عَنْ عُقْبَةَ بْنِ عَامِرٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَلَمْ تَرَ آيَاتٍ أُنْزِلَتِ اللَّيْلَةَ لَمْ يُرَ مِثْلُهُنَّ قَطُّ ‏{‏ قُلْ أَعُوذُ بِرَبِّ الْفَلَقِ‏}‏ وَ ‏{‏ قُلْ أَعُوذُ بِرَبِّ النَّاسِ‏}‏ ‏"‏ ‏.‏
Reference : Sahih Muslim 814a
In-book reference : Book 6, Hadith 319
USC-MSA web (English) reference : Book 4, Hadith 1774
  (deprecated numbering scheme)