Musnad Ahmad 30:893

Abu Amir narrated to us, Malik narrated to us, from Abdullah ibn Abd al-Rahman, from Ibn Hunayn, from Abu Hurayrah: That the Prophet, peace and blessings of Allah be upon him, heard a man reciting "Say, He is Allah, [who is] One." He said, "It has become incumbent." They said, "O Messenger of Allah, what has become incumbent?" He said, "Paradise has become incumbent for him."