The Prophet ﷺ said, "Is any one of you incapable of reciting a third of the Quran in a single night?"This was difficult for his companions, and they said, "Who among us could manage that?"He replied, "Let him recite 'Say, He is Allah, the One.' That is a third of the Quran."
حدثنا عبد الله بن محمد قال أبو عبد الرحمن وسمعته أنا من عبد الله بن محمد بن أبي شيبة حدثنا أبو خالد الأحمر عن الأعمش عن الضحاك المشرقي
عن النبي ﷺ أنه قال أيعجز أحدكم أن يقرأ ثلث القرآن في ليلة قال فشق ذلك على أصحابه فقالوا من يطيق ذلك قال يقرأ قل هو الله أحد فهي ثلث القرآن
Its isnad is sahih. Dahhak Mishraqi is Ibn Sharahil, truthful, and his hadith is in the Sahihayn. Abu Khalid Ahmar is Sulayman bin Hayan, truthful but prone to error, and his hadith is found among the community of scholars.The hadith was narrated by Bukhari, 6/233, in the chapters on the merits of the Quran, and all of them narrated it regarding the merit of "Say, He is Allah, One" [Ikhlas 112:1], and by Muslim, 1/556, no. 811, in the chapter on the prayer of travelers, and by Tirmidhi, who graded it hasan, 5/167, no. 2896, and by Darimi, 2/553, no. 3437, both of them narrating it as Bukhari did.