Musnad Abu Ya'la al-Mawsili 160:179

Abu Hisham al-Rifa'i narrated to us, Ibn Fudayl narrated to us, Abdullah bin Sa'id bin Abi Sa'id narrated to us, from his grandfather, from Abu Hurayrah, from the Prophet (peace be upon him), who said: "Indeed, Allah is As-Salam (the Peace), so do not begin with anything before Him. If it is said to you, 'As-salamu 'alaykum' (Peace be upon you), then say, 'As-salamu 'alaykum'."