Musnad Ahmad 132:7

Hassan narrated to us, Ibn Lahi'ah narrated to us, Zabban narrated to us, from Sahl ibn Mu'adh, from his father, from the Messenger of Allah (may Allah bless him and grant him peace) that he said: "It is a duty upon one who stands at a gathering to greet them, and it is a duty upon one who stands up from a gathering to greet." A man stood up while the Messenger of Allah was speaking and did not greet. The Messenger of Allah (may Allah bless him and grant him peace) said: "How quickly he has forgotten."