28 CFR 16.53: Use and collection of social security numbers.
Each component shall ensure that employees authorized to collect information are aware:
(a) That individuals may not be denied any right, benefit, or privilege as a result of refusing to provide their social security numbers, unless the collection is authorized either by a statute or by a regulation issued prior to 1975; and
(b) That individuals requested to provide their social security numbers must be informed of:
(1) Whether providing social security numbers is mandatory or voluntary;
(2) Any statutory or regulatory authority that authorizes the collection of social security numbers; and
(3) The uses that will be made of the numbers.