Musnad Ahmad 962:152

Abu al-Mughirah narrated to us, Ma'an ibn Rifa'ah narrated to us, Ali ibn Yazid narrated to me, from al-Qasim, from Abu Umamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by a man while he was reciting, "Say, 'He is Allah, [who is] One'" [Al-Ikhlas: 1]. He said, "This has made it obligatory," or "Paradise has become obligatory for this one."