Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said: Qatadah bin al-Nu'man (may Allah be pleased with him) informed me that a man stood in prayer in the time of the Prophet (peace and blessings be upon him) and recited "Say: He is Allah, the One" [Surah al-Ikhlas: 1] over and over, reciting nothing besides it. The narrator said: When morning came, a man said to the Messenger of Allah (peace and blessings be upon him): "O Messenger of Allah! A man stood in prayer at night and kept repeating 'Say: He is Allah, the One. Allah, the Self-Sufficient. He neither begets nor was He begotten, and there is none comparable to Him' [Surah al-Ikhlas: 1-4], reciting nothing besides it." It was as though that man considered the surah too little. The Messenger of Allah (peace and blessings be upon him) said: "By the One in Whose hand is my soul, this surah is equal to a third of the Qur'an."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4763