The Prophet ﷺ asked, "Is any one of you incapable of reciting a third of the Quran every night?"We said, "Messenger of Allah, who could manage that?"He replied, "Say, He is Allah, the One" [Ikhlas 112:1] equals a third of the Quran.
أن النبي ﷺ قال: أيغلب أحدكم أن يقرأ ثلث القرآن كل ليلة؟ قلنا: ومن يطيق ذلك يا رسول الله؟ قال: {قل هو الله أحد} ثلث القرآن.
Tayalisi (651), Ibn Majah (3789), and Ahmad (17106) all narrated it from the hadith of Abu Qays, from Amr bin Maymun, from Abu Masud, and he mentioned it.Its chain of transmission is hasan because of Abu Qays, who is Abdul-Rahman bin Tharwan Al-Kufi. There is disagreement about him, but his hadith is hasan when it does not conflict with others. The hadith has other routes of transmission, but the one I have mentioned is the best of them.