Musnad Ahmad 32:570

Husayn ibn Muhammad narrated to us, al-Mubarak narrated to us, from Thabit, from Anas, who said: A man said: "O Messenger of Allah, I truly love this surah: 'Say, He is Allah, [who is] One.'" The Messenger of Allah, peace and blessings of Allah be upon him, said: "Your love for it has admitted you into Paradise."