It is narrated from Sayyiduna Shaybah (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) said: "When one of you arrives at a gathering, if room is made for him, let him sit; otherwise, let him look for the most spacious place he sees and sit there."
Hadith Referenceالمعجم الكبير للطبراني / باب الشين / 7166