The Messenger of Allah ﷺ said, "Whoever recites 'Say, He is Allah, the One' [Ikhlas 112:1] has recited the equivalent of one third of the Quran."
قال رسول الله ﷺ: من قرأ ب {قل هو الله أحد} كأنما قرأ بثلث القرآن.
Ahmad (21275) and Abu Ubayd in Fadail Quran (p. 268) both narrated it from Hushaym, who said, Husayn informed us, from Hilal bin Yasaf, from Abdul-Rahman bin Abu Layla, from Ubayy bin Kaab or from a man among the Ansar, and he related it. Its isnad is sahih. Hushaym is Ibn Bashir, and Husayn is Ibn Abdul-Rahman Sulami. Haythami mentioned it in Majma (7/147) and said, "Ahmad narrated it, and its narrators are the narrators of the sahih."His statement, "or a man among the Ansar," does not affect this because this doubt does not cause harm, for the man among the Ansar is also a companion. Moreover, it has come through other routes from Ubayy bin Kaab without this doubt, and that is more likely correct, since Ubayy bin Kaab was among the greatest reciters of the companions. In the same meaning, it has also been narrated from Abu Ayub Ansari, and from Umm Kulthum bint Uqba and others among the companions, though their isnads contain defects.The meaning of the recitation of "Say, He is Allah, One" [Ikhlas 112:1] being equivalent to a third of the Quran is that the Quran encompasses the oneness of Allah, the message of the messengers, the Day of Judgment, and their details. Surah Ikhlas is entirely devoted to explaining the oneness of Allah in the most beautiful expressions and the shortest words. So whoever recites it has, as it were, come to know the meaning of the oneness of Allah, which is one of these three matters, and his reward will be like that of one who recited a third of the Quran, except that it does not equal the reward of one who spends the entire night reciting the Quran, for the reward of reciting the Quran has different ranks with Allah, may He be exalted.The scholars have differed greatly in explaining and interpreting this hadith, to the point that it is difficult to enumerate all their views, so much so that Ibn Abdul-Barr said, "Remaining silent about this matter is better than speaking about it."