Sahih al-Bukhari 6643

Abdullah ibn Maslamah narrated to us, from Malik, from Abd al-Rahman ibn Abdullah ibn Abd al-Rahman, from his father, from Abu Sa'id, that a man heard another man repeatedly reciting, ‘Say: He is Allah, the One.’ When morning came, he went to the Messenger of Allah, peace and blessings be upon him, and told him about it, as though he considered the recitation too little. The Messenger of Allah, peace and blessings be upon him, said: ‘By the One in whose hand is my soul, it is equal to one third of the Quran.’