Musnad Abu Ya'la al-Mawsili 42:44

Muhammad ibn Isma'il ibn Abi Samina narrated to us, Abu Khalid al-Ahmar narrated to us, from al-A'mash, from al-Dahhak al-Mashriqi, from Abu Sa'id al-Khudri, from the Prophet (peace be upon him), who said: "Say, 'He is Allah, [who is] One' (Surah al-Ikhlas) is equivalent to one-third of the Qur'an."