The Prophet ﷺ said, "Is any of you unable to recite a third of the Quran in a single night?"They asked, "How can one recite a third of the Quran?"He replied, "Say, He is Allah, the One" [Ikhlas 112:1] is equivalent to a third of the Quran."
عن النبي ﷺ قال: أيعجز أحدكم أن يقرأ في ليلة ثلث القرآن. قالوا: وكيف يقرأ ثلث القرآن؟ قال: {قل هو الله أحد} يعدل ثلث القرآن.
Muslim narrated it in Fadail Quran (811:259) through several routes from Yahya bin Saeed, from Shuba, from Qatada, from Salim bin Abu Jaad, from Madan bin Abu Talha, from Abu Darda, as mentioned.He also narrated it through another route from Qatada with this same chain (811:260), with the wording, "Allah divided the Quran into three parts, and He made "Say, He is Allah, One" a part of the parts of the Quran."