I observed the Messenger of Allah ﷺ twenty times, and he would recite in the two units of prayer after Maghrib and in the two units before Fajr, "Say, O you disbelievers" [Kafirun 109:1] and "Say, He is Allah, the One" [Ikhlas 112:1].
أخبرنا الفضل بن سهل قال حدثني أبو الجواب قال حدثنا عمار بن رزيق عن أبي إسحق عن إبراهيم بن مهاجر عن مجاهد
رمقت رسول الله ﷺ عشرين مرة يقرأ في الركعتين بعد المغرب وفي الركعتين قبل الفجر قل يا أيها الكافرون وقل هو الله أحد
Its isnad is daif, and it is in Kubra, no. 1064. Tirmidhi, no. 417, and Ibn Majah, no. 1149, and others also narrated it, from the hadith of Abu Ishaq from Mujahid from Ibn Umar, in this form. This is tadlis. Part of the hadith has supporting narrations in Muslim, no. 726, and elsewhere.