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