Health and Safety Code §1371.1(a)(1)

(a)(1) Whenever a health care service plan, including a specialized health care service plan, determines that in reimbursing a claim for provider services an institutional or professional provider has been overpaid, and then notifies the provider in writing through a separate notice identifying the overpayment and the amount of the overpayment, the provider shall reimburse the health care service plan within 30 working days of receipt by the provider of the notice of overpayment unless the overpayment or portion thereof is contested by the provider in which case the health care service plan shall be notified, in writing, within 30 working days.  The notice that an overpayment is being contested shall identify the portion of the overpayment that is contested and the specific reasons for contesting the overpayment.