He replied, “‘Say, He is Allah, the One’ {1} is equivalent to one-third of the Quran.” Reported by al-Bukhārī.

Another report from al-Bukhārī, on the authority of Qatāda ibn al-Nu‘mān, states that a man in the Prophet’s lifetime, may Allah bless him and grant him peace, would recite “‘Say, He is Allah, the One’ {1}” at dawn, repeating it without reciting anything else. When morning came, a man approached the Prophet, may Allah bless him and grant him peace, and said, “O Messenger of Allah, so-and-so spent the night reciting ‘Say, He is Allah, the One’ {1} before dawn, repeating it without reciting anything else.” The man seemed to regard this as little. The Prophet, may Allah bless him and grant him peace, said, “By Him in whose hand is my soul, it is indeed equivalent to one-third of the Quran.”

Muslim narrated that Abū Hurayrah said: “The Messenger of Allah, may Allah bless him and grant him peace, came out to us and said, ‘I shall recite to you one-third of the Quran.’ He then recited ‘Say, He is Allah, the One’ {1}