20 CFR 230.2 - Loss of annuity for month in which compensated service is rendered.
If an individual in receipt of an annuity renders compensated service, he shall not be paid an annuity with respect to any month in which such service is rendered to:
(a) An employer;
(b) Any person whether or not an employer by whom he was most recently employed when his annuity begins to accrue;
(c) Any person with whom he held, at the time the annuity begins to accrue, any rights to return to service;
(d) Any person with whom he ceased service in order to have his annuity begin to accrue.