Musnad Ahmad 30:734

Qaran ibn Tamam narrated to us, from Muhammad ibn ‘Ajlan, from Sa‘id ibn Abi Sa‘id, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you comes to a gathering, let him offer the greeting of peace. Then, if it seems appropriate to him to sit, let him sit. And when he stands up, let him offer the greeting of peace, for the first is not more obligatory than the last."