According to another version, he (Sallallaahu ‘alaihi wasallam) said to his Companions, “Is anyone of you incapable of reciting one-third of the Qur’an in one night?” They considered it burdensome and said: “O Messenger of Allah, which of us can afford to do that?” He (Sallallaahu ‘alaihi wasallam) said, “Surat Al-Ikhlas [Say: He is Allah (the) One] is equivalent to one-third of the Qur’an.”
One reading of Surat Al-Ikhlas has return and reward equal to the recitation of one-third of the Qur’an. This Surah mentions the Oneness of Allah. This is enough to make one realize how much Allah (SWT) likes the recitation and remembrance of His Oneness. This is the reason, He has declared Shirk (polytheism) an absolutely unpardonable sin.
Riyad-us-Saliheen – Hadith No. 1010