Maliks Muwatta Book 15, Hadith Number 17.
Section : About Reciting Surat al-Ikhlas and Surat al-Mulk.
Yahya related to me from Malik from Abd ar-Rahman ibn Abdullah ibn Sasaca from his father that Abu Said al-Khudri heard a man reciting Surat al-Ikhlas (Sura 112), repeating it over and over again. In the morning he went to the Messenger of Allah, may Allah bless him and grant him peace, and mentioned it to him, as if he thought little of it. The Messenger of Allah, may Allah bless him and grant him peace, said, “By the One in whose hand my self is, it is equal to one third of the Qur’an.”