Surat Al-Ikhlas, One third of the Qur’an
The Prophet (sal Allahu alaihi wa sallam) said about Surat Al-Ikhlas (Chapter 112): “By Him in Whose Hand my soul is, it is equivalent to one-third of the Qur’an.”
In other narrations he (sal Allahu alaihi wa sallam) 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 (sal Allahu alaihi wa sallam) said, “Surat Al-Ikhlas [Say: He is Allah (the) One] is equivalent to one-third of the Qur’an.” [Sahih Al-Bukhari]
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 likes the recitation and remembrance of His Oneness. This is the reason, He has declared Shirk (polytheism) an absolutely unpardonable sin.
And Allah knows best!