Musnad Abu Ya'la al-Mawsili 160:170

Masruq ibn al-Marzuban narrated to us, Abd al-Salam ibn Harb narrated to us, from Abdullah ibn Sa'id, from his grandfather, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you intends to give the greeting (salam), let him say: 'As-salamu 'alaykum' (Peace be upon you), for indeed Allah is As-Salam (The Peace). And do not begin anything before Allah."