Assuming what you actualy want to know is what file sets the prompt to user@hostname the answer is: probably /etc/profile
However that may not be the question and answer you need.
At log in a number of scripts are run all of which call other commands. Any of which could be the problem.
Have a look at the following:
All files in /etc/profile.d
The following files in your user's home directory: .profile, .bashrc, .bash_aliases
There may be others, especially if you're using a different shell. I've no idea if systemd fires anything on login.
Attempts to contact me outside of these forums will be ignored unless signed in triplicate, sent in, sent back, queried, lost, found, subjected to public enquiry, lost again, and finally buried in soft peat for three months and recycled as firelighters