The Prophet ﷺ would recite in the two (supererogatory) rak'ahs of the dawn prayer: "Say, O unbelievers" [Surat al-Kafiroon:1] and "Say: He is Allah, the one" [Surat al-Ikhlas:1] (Using translation from Abū Dāʾūd 1256)  

أَنَّ رَسُولَ اللهِ ﷺ قَرَأَ فِي رَكْعَتَيِ الْفَجْرِ قُلْ يَا أَيُّهَا الْكَافِرُونَ وَقُلْ هُوَ اللهُ أَحَدٌ  

[Machine] "I asked the Messenger of Allah ﷺ : 'I have come to you, O Messenger of Allah, to teach me something to say when I go to sleep.' He replied: 'When you go to bed, recite Surah Al-Kafirun, then sleep. For it is an immunity from shirk (polytheism)."  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «فَمَجِيءٌ مَا جَاءَ بِكَ» قُلْتُ جِئْتُ يَا رَسُولَ اللهِ لِتُعَلِّمَنِي شَيْئًا أَقَوْلُهُ عِنْدَ مَنَامِي قَالَ «إِذَا أَخَذْتَ مَضْجَعَكَ فَاقْرَأْ قُلْ يَا أَيُّهَا الْكَافِرُونَ ثُمَّ نَمْ عَلَى خَاتِمَتِهَا فَإِنَّهَا بَرَاءَةٌ مِنَ الشِّرْكِ»