Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I observed the Prophet (peace and blessings be upon him) twenty-five times; in the two rak'ahs before Fajr and the two rak'ahs before Maghrib he (peace and blessings be upon him) would recite "Qul ya ayyuha al-kafirun" and "Qul huwa Allahu ahad".
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13530