Код:

<script type="text/javascript">
function addLink() {
var body_element = document.getElementsByTagName('body')[0];
var selection;
selection = window.getSelection();
var pagelink = " (источник: <a href='"+document.location.href+"'>"+document.location.href+"</a>";
var copytext = selection + pagelink;
var newdiv = document.createElement('div');
newdiv.style.position='absolute';
newdiv.style.left='-99999px';
body_element.appendChild(newdiv);
newdiv.innerHTML = copytext;
selection.selectAllChildren(newdiv);
window.setTimeout(function() {
body_element.removeChild(newdiv);
},0);
}
document.oncopy = addLink;
</script>