American Bar Association Model Rules of Professional Conduct - Rule 5.6. Restrictions on Rights to Practice
A lawyer shall not participate in offering or making:
(a) a partnership, shareholders, operating, employment, or other similar type of agreement that restricts the right of a lawyer to practice after termination of the relationship, except an agreement concerning benefits upon retirement; or
(b) an agreement in which a restriction on the lawyer's right to practice is part of the settlement of a client controversy.