No man should move another from the place he is sitting then sit in it. Instead, make room and spread out
Authentic hadithعن عبد الله بن عمر رضي الله عنهما قال: قال رسول الله صلى الله عليه وسلم : «لا يُقِيمُ الرجلُ الرجلَ من مَجْلِسِهِ، ثم يجلس فيه، ولكن تَفَسَّحُوا، وتَوَسَّعُوا».
‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "No man should move another from the place he is sitting then sit in it. Instead, make room and spread out."
Explanation
This Hadīth includes two of the etiquettes related to gatherings: 1. It is not permissible for one person to make another leave the place he is sitting in - which he got to before him - and then sit in it. 2. It is obligatory for those who are present to spread out to find a place for a newcomer among them. Allah, the Exalted, says (what means): {O you who believe! When you are told to make room in the assemblies, spread out and make room and Allah will give you ample room.} [Sūrat al-Mujādilah: 11]
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).