Musnad Ahmad 30:2536

Yahya narrated to us, from Ibn ‘Ajlan, who said: Sa‘id narrated to me, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "When one of you reaches a gathering, let him offer the greeting of peace. Then, if it seems appropriate to him to sit, let him sit. Then, if he stands up while the people are still sitting, let him offer the greeting of peace, for the first [greeting] is not more entitled than the last."