A subform does not have a ControlSource property. Are you wanting the
SourceObject property (which is the name of the form that is being used as
the subform)?
Assuming yes, this reference can be used in the module for the main form:
Neither a subform control nor a form object displayed in a
subform control have a control source property. If you
meant the record source property, then the syntax would be:
Me.[subform control].Form.RecordSource