Purity of Faith (Al Ikhlas)
In The Shade Of The Quran by Imam Syed Qutb

This short surah is "equivalent to one third of the Qur'an". al Bukhari, the leading traditionist, was told about someone who had heard a man reciting, "Say: He is Allah ..." repeatedly and had gone to the Prophet the following morning and told him disapprovingly about what he had heard. The Prophet commented, "I swear by Him in Whose hand is my soul that it (the surah) is equivalent to one third of the Qur'an."