The Prophet ﷺ said to Nawfal, "Recite 'Say, O you disbelievers' [Kafirun 109:1] and then go to sleep upon its conclusion, for it is a disavowal of polytheism."
حدثنا النفيلي حدثنا زهير حدثنا أبو إسحق
أن النبي ﷺ قال لنوفل اقرأ قل يا أيها الكافرون ثم نم على خاتمتها فإنها براءة من الشرك
Sourcing, hasan. Nasai reported it in Al-Kubra, no. 10637, from the hadith of Zuhayr through this route. Ibn Hibban graded it sahih, no. 2363, 2364, as did Hakim, 1/565, 3/538, and Dhahabi agreed with him. It has another version with Tirmidhi, no. 3403, and it is hasan on the basis of supporting evidence. This hadith also has another portion, "and the Prophet handed over his stepdaughter to him," and so on, which Bukhari mentioned without a full chain in his Sahih just before hadith no. 5106.