Instructions for  Form 1120S, Page 19, Instructions for Line 17, reporting requirements for for single owner Sub S Retirement.

• Form 5500-EZ, Annual Return of A One-Participant (Owners/
Partners and Their Spouses) Retirement Plan or A Foreign Plan.
File this form for a plan that only covers the owner (or the owner
and the owner's spouse) but only if the owner (or the owner and
the owner's spouse) owns the entire business.