A quick and dirty definition of User Story:
"As a <role>, I want <goal/desire> so that <benefit>"
In this commonly accepted definition there is little space for defining business rules, constraints or user input.
Trivial example just to illustrate:
"As a <librarian>, I want to <register new books> so that <students can find their availability online>"
In this silly example, where would one define the fields needed when registering a book? Should it be written anywhere? Or should the required business rules be passed as word of mouth by the Product Owner?