29 U.S.C. § 658 : US Code - Section 658: Citations

    (a) Authority to issue; grounds; contents; notice in lieu of
      citation for de minimis violations
      If, upon inspection or investigation, the Secretary or his
    authorized representative believes that an employer has violated a
    requirement of section 654 of this title, of any standard, rule or
    order promulgated pursuant to section 655 of this title, or of any
    regulations prescribed pursuant to this chapter, he shall with
    reasonable promptness issue a citation to the employer. Each
    citation shall be in writing and shall describe with particularity
    the nature of the violation, including a reference to the provision
    of the chapter, standard, rule, regulation, or order alleged to
    have been violated. In addition, the citation shall fix a
    reasonable time for the abatement of the violation. The Secretary
    may prescribe procedures for the issuance of a notice in lieu of a
    citation with respect to de minimis violations which have no direct
    or immediate relationship to safety or health.
    (b) Posting
      Each citation issued under this section, or a copy or copies
    thereof, shall be prominently posted, as prescribed in regulations
    issued by the Secretary, at or near each place a violation referred
    to in the citation occurred.
    (c) Time for issuance
      No citation may be issued under this section after the expiration
    of six months following the occurrence of any violation.