Soon Hing v. Crowley,
113 U.S. 703 (1885)

nThe petition alleges that it was adopted owing to a feeling of antipathy and hatred prevailing in the city and county of San Francisco against the subjects of the emperor of China resident therein, and for the purpose of compelling those who engaged in the laundry business to abandon their lawful avocation and residence there, and not for any sanitary, police, or other legitimate purpose.  There is nothing, however, in the language of the ordinance, or in the record of its enactment, which in any respect tends to sustain this allegation. . . .  [E]ven if the motives of the supervisors were as alleged, the ordinance would not be thereby changed from a legitimate police regulation, unless in its enforcement it is made to operate only against the class mentioned; and of this there is no pretense.