I observed the Prophet ﷺ for a month, and in the two units of prayer before Fajr he would recite "Say, O disbelievers" [Kafirun 109:1] and "Say, He is Allah, the One" [Ikhlas 112:1].
حدثنا محمود بن غيلان وأبو عمار قالا حدثنا أبو أحمد الزبيري حدثنا سفيان عن أبي إسحق عن مجاهد
رمقت النبي ﷺ شهرا فكان يقرأ في الركعتين قبل الفجر بقل يا أيها الكافرون وقل هو الله أحد
[Hasan] Ibn Majah also recorded it in Iqamat al-Salawat, chapter on what is recited in the two units of prayer before Fajr, no. 1149, from the hadith of Abu Ahmad Muhammad bin Abdullah Zubayri, with this wording, and he is hasan al-hadith. Sufyan here is Thawri. The hadith has corroborating narrations in Muslim, no. 726, and elsewhere.On this subject there are also narrations from Ibn Masud [coming up: 431], Anas [Bazzar (Kashf): 1/1338, no. 70], Abu Hurairah [Muslim, no. 726], Ibn Abbas [Muslim, no. 727], Hafsa [Bukhari, no. 618], and Ayesha [Bukhari, no. 619, and Muslim, no. 724].