Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that a man heard another man reciting « قُلْ هُوَ اللَّـهُ أَحَدٌ», and he was reciting it repeatedly. The man who heard this mentioned it to the Messenger of Allah (peace and blessings be upon him) in the morning, as if he considered this to be a small act. The Messenger of Allah (peace and blessings be upon him) said: "By Him in Whose hand is my soul! Indeed, this (Surah Ikhlas) is equal to one third of the Qur'an."