Narrated Abu Hurairah (May Allah be pleased with him):
Messenger of Allah (ﷺ) said: Get together for I am going to recite one-third of the Qur’an before you. And those who could get together gathered there. Then the Messenger of Allah (ﷺ) came out and recited:” Say: He, Allah, is One.” He then entered (his house). Some of us said to the others: Perhaps there has been some news from the heaven on account of which he has gone inside (the house). The Apostle of Allah (ﷺ) again came out and said: I told you that I was going to recite one-third of the Qur’an; keep in mind, this (Surah Ikhlas) is equivalent to one-third of the Qur’an. (Sahih Muslim 812 a, Book 6, Hadith 316)
Narrated Abu Said Al-Khudri (May Allah be pleased with him):
A man heard another man reciting (Surat-Al-Ikhlas) ‘Say He is Allah, (the) One.’ (112:1) repeatedly. The next morning he came to Allah’s Messenger (ﷺ) and informed him about it as if he thought that it was not enough to recite. On that Allah’s Messenger (ﷺ) said, “By Him in Whose Hand my life is, this Surah is equal to one-third of the Qur’an!” (Sahih Al Bukhari, Vol. 6, Book 61, Hadith 533)
‘Aishah (May Allah be pleased with her) reported:
Messenger of Allah (ﷺ) appointed a man in charge of an army unit who led them in Salat (prayer); he always concluded his recitation with Surat Al-Ikhlas: “Say (O Muhammad (ﷺ)): ‘He is Allah, (the) One. Allah-us-Samad (Allah – the Self-Sufficient). He begets not, nor was He begotten. And there is none equal or comparable to Him.”‘ (112:1-4) Upon their return to Al-Madinah, they mentioned this to Messenger of Allah (ﷺ), who said, “Ask him why he does so?” He was asked and he said, “This Surah contains the Attributes of Allah, the Gracious, and I love to recite it. Messenger of Allah (ﷺ) then told them, “Tell him that Allah loves him”. (Riyad as-Salihin [Bukhari & Muslim] Book 1, Hadith 388)