Musnad Ahmad 31:128

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, from al-Harith ibn Yazid, from Abu al-Haytham, from Abu Sa'id al-Khudri, who said: Qatadah ibn al-Nu'man spent the night reciting the whole night: "Say, He is Allah, [who is] One." He mentioned that to the Prophet, peace be upon him. The Prophet, peace be upon him, said: "By the One in whose hand is my soul, it is equivalent to half of the Qur'an—or a third of it."