Musnad Abu Ya'la al-Mawsili 117:61

Mus'ab ibn 'Abd Allah al-Zubayri narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from 'Ubayd Allah ibn 'Umar, from Thabit al-Bunani, from Anas ibn Malik: That a man used to persist in reciting "Say, He is Allah, [who is] One" in the prayer in every surah while he was leading his companions in prayer. The Messenger of Allah (may Allah bless him and grant him peace) said to him, "What compels you to [recite] this surah?" He said, "Indeed, I love it." He said, "Your love for it has admitted you into Paradise."