General Behavior (Kitab Al-Adab)
Dawud :: Book 41 : Hadith 5189
Narrated AbuHurayrah:
The Prophet (peace_be_upon_him) said: When one of you comes to an assembly, he should give a salutation and if he feels inclined to get up, he should give a salutation, for the former is not more of a duty than the latter.