CA Ins Code §10123.145(a)(1)

(1) Whenever an insurer issuing group or individual policies of disability insurance which covers hospital, medical, or surgical expenses 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 insurer 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 insurer 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.