Musnad Abu Ya'la al-Mawsili 160:171

Sahl ibn Zanjalah narrated to us, al-Walid narrated to us, I heard Ibn ‘Ajlan mentioning from Sa‘id al-Maqburi, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When one of you comes to a gathering, let him offer greetings (salam). Then, if he stands up (to leave), let him offer greetings (salam), for the first is not more deserving than the latter."