A man heard another man reciting "Say, He is Allah, the One" repeatedly. When morning came, he went to the Messenger of Allah ﷺ and mentioned it to him, as though he considered it a small thing.The Prophet ﷺ said, "By the One in whose hand my soul is, it equals one third of the Quran."
حدثنا القعنبي عن مالك عن عبد الرحمن بن عبد الله بن عبد الرحمن عن أبيه
أن رجلا سمع رجلا يقرأ قل هو الله أحد يرددها فلما أصبح جاء إلى رسول الله ﷺ فذكر له وكأن الرجل يتقالها فقال النبي ﷺ والذي نفسي بيده إنها لتعدل ثلث القرآن
Bukhari narrated it in "Virtues of the Quran," chapter on the virtue of "Say, He is Allah, One," no. 5013, from the hadith of Malik through this route, and it is also found in the Muwatta (Yahya's recension), 1/208, and Qanabi's recension, pp. 142–143.