Does IDREF require a matching ID?

This may be a trivial question in this audience, but if you have an
attribute defined in the schema as "xs:IDREF", does there also have to
be at least one attribute defined in the schema as "xs:ID", and a
similar relationship in the XML instance document, with the IDREF value
matching at least one ID attribute?  If this relationship does not hold
true, does this mean that the document is not valid?

If so, can you point me somewhere in the specs that state this?