Musnad Abu Ya'la al-Mawsili 76:1

Abu Ma'mar al-Hudhali Isma'il ibn Ibrahim narrated to us, Isma'il ibn Ja'far narrated to us, from Malik ibn Anas, from 'Abd al-Rahman ibn 'Abd Allah ibn 'Abd al-Rahman ibn Sa'sa'ah, from his father, from Abu Sa'id al-Khudri, who said: My brother Qatadah ibn al-Nu'man informed me that a man stood during the time of the Prophet, may Allah bless him and grant him peace, reciting in the early morning: "Say, 'He is Allah, [who is] One,'" (Al-Ikhlas), repeating it and not adding anything to it. When morning came, a man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, such-and-such a person stood during the night and recited in the early morning: 'Say, "He is Allah, [who is] One," Allah, the Eternal Refuge. He neither begets nor is born, nor is there to Him any equivalent,'" (Al-Ikhlas), repeating it and not adding anything to it, as if the man considered it too little. The Prophet, may Allah bless him and grant him peace, said: "By the One in whose hand is my soul, it is indeed equal to one-third of the Quran."