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The Prophet (ﷺ) ﷺ asked: “Is any one of you unable to recite one-third of the Qur'ān in one night? They said: How could anyone read one-third of the Qur'ān (in one night)? He said: Qul huwa Allāhu aḥad (Sūrat al-Ikhlāṣ) is equivalent to one-third of the Qur'ān.” Reference: Sahih Muslim 811a.
Source: sunnah.com
Other translations
Arabic
عَنْ أَبِي الدَّرْدَاءِ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " أَيَعْجِزُ أَحَدُكُمْ أَنْ يَقْرَأَ فِي لَيْلَةٍ ثُلُثَ الْقُرْآنِ " قَالُوا : وَكَيْفَ يَقْرَأْ ثُلُثَ الْقُرْآنِ؟ قَالَ : " قُلْ هُوَ اللَّهُ أَحَدٌ تَعْدِلُ ثُلُثَ الْقُرْآنِ " .
sunnah.com