Musnad Ahmad 32:492

Abu al-Nadr narrated to us, al-Mubarak narrated to us, from Thabit al-Bunani, from Anas ibn Malik, who said: A man came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "I love this surah: 'Say, He is Allah, [who is] One.'" The Messenger of Allah, may Allah bless him and grant him peace, said: "Your love for it has admitted you into Paradise."