Is it considered a sin to separate two people sitting in the mosque, and is proof required for that?

It is forbidden to separate two people on Friday or any other day, except with their permission. The Prophet, peace and blessings be upon him, prohibited that, as well as one man making another rise from his seat to sit in it, for that is harm and injustice. Whoever needs to sit between two people must ask their permission. If they permit it, then it is permissible; otherwise, it is not. If one wishes to advise a Muslim, there is no condition to mention the proof, but one should only speak what they know to be true; as Allah, the Most High, says: ﴿And do not pursue that of which you have no knowledge. Indeed, the hearing, the sight, and the heart - about all those [one] will be questioned.﴾