It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that he said: "The Messenger of Allah (peace and blessings be upon him) prayed two units (rak'ahs), in which he recited only Surah al-Fatihah (Umm al-Kitab), and he did not add anything to it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12960