You have to set the range on a property. The integer is defined in XMLSchema. For instance:
<rdf:Property rdf:ID="testRelation">
<rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#integer"/>
</rdf:Property>
Note that this is not a true restriction. As you should know in RDF anyone can say anything about everything. That means that one person can decide that the range also includes doubles and then this will be true as well.
If you are looking at some kind of sanity check then you will have to do this outside rdf (or use an inferences to spot inconsistencies).