Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I cannot count how many times I heard the Prophet (peace and blessings be upon him) reciting in the two rak'ahs of Maghrib and of Fajr: "Say: O disbelievers" [Surah al-Kafirun: 1] and "Say: He is Allah, the One" [Surah al-Ikhlas: 1].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4878
Ibn Umar (may Allah be pleased with them both) said: I heard the Prophet (peace and blessings be upon him) more than twenty times "reciting in the two rak'ahs after Maghrib and the two rak'ahs before Subh 'Say: O disbelievers' [Surah al-Kafirun: 1] and 'Say: He is Allah, the One' [Surah al-Ikhlas: 1]."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4879