ENOfficial
Farwa b. Naufal quoted his father as saying that he had asked God's messenger to teach him something to say when he went to bed, and was told to recite, “Say, O infidels”, for it is a declaration of freedom from polytheism. Tirmidhī, Abū Dāwūd and Dārimī transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ فَرْوَةَ بْنِ نَوْفَلٍ عَنْ أَبِيهِ: أَنَّهُ قَالَ: يَا رَسُولَ اللَّهِ عَلِّمْنِي شَيْئًا أَقُولُهُ إِذَا أَوَيْتُ إِلَى فِرَاشِي. فَقَالَ: «اقْرَأْ (قُلْ يَا أَيُّهَا الْكَافِرُونَ) فَإِنَّهَا بَرَاءَةٌ مِنَ الشِّرْكِ» . رَوَاهُ التِّرْمِذِيّ وَأَبُو دَاوُد والدارمي
sunnah.com