Abu Sa’id reported that the Prophet, peace be upon him, asked, “Can anyone of you recite a third of the Qur’an during the night?” The Companions considered this rather difficult and they said, “Who among us can do so, O Prophet of Allah?” Thereupon the Prophet, peace be upon him, said, “(Say:) He is Allah, the One and Only; Allah, the Eternal, Absolute; He begetteth not, nor is he begotten; and there is none like unto Him.”
(The reference here is to recitation of Surah 112 (Suratul Ikhlas) of the Qur’an, which the Prophet, peace be upon him, described as equivalent to reciting one-third of the Qur’an. This hadith is reported by Bukhari, Muslim, and Nasa’i)