Joseph Nee
The document which shows that your are inside Thailand is called the passport.
Just make a copy of his Admitted until stamp.
Not sure what happened, but the post disappeared.
TLDR : Answer Summary
The main document demonstrating your legal stay in Thailand is your passport, specifically the admission stamp located within. It is advised to make a copy of the stamp for your records. Additionally, the discussion included a mention of handling rude comments in visa advice forums, emphasizing that such pages are intended for assistance with Thai visa issues.