Is this inside of the same `control-group` as sources for any particular reason?  Could you move it to a separate one so that it sits on one line?

I *think* this would work:

    <div class="control-group">
      <% f.label :propagate_immediately, :class => 'control-label' do %>
        <%= f.check_box :propagate_immediately %>
      <% end %>
    </div>

You may need a tiny but of CSS so that it's vertically centered.