Musnad Abu Ya'la al-Mawsili 140:33

Qatan ibn Nusayr al-Ghubari narrated to us, 'Ubayyis ibn Maymun al-Qurashi narrated to us, Yazid al-Raqashi narrated to us, from Anas ibn Malik, who said: I heard the Prophet, may Allah bless him and grant him peace, saying: "Is one of you not able to recite 'Say, He is Allah, [who is] One' (Al-Ikhlas) three times in a night? For it is equivalent to one-third of the Qur'an."