I observed the Prophet ﷺ for a month, and in the two units of prayer before Fajr he would recite "Say, O disbelievers" and "Say, He is Allah, the One."
حدثنا أبو أحمد الزبيري حدثنا سفيان عن أبي إسحاق عن مجاهد
رمقت النبي ﷺ شهرا فكان يقرأ في الركعتين قبل الفجر قل يا أيها الكافرون وقل هو الله أحد
Its isnad is sahih. It is an abridgment of 5215, and we noted at 4763 that Tirmidhi related part of it via Abu Ahmad Zubayri from Thawri. This is the narration of Abu Ahmad. See also 4909.