Cal. Rev. & Tax. Code § 8104

In order to establish the validity of any claim the Controller may, upon demand, examine the books and records of the claimant for that purpose. The failure of the claimant to accede to that demand constitutes a waiver of all right to the refund claimed on account of the transactions questioned. The examination may be made either through employees of the office of the Controller or of the office of the board. Supporting evidence of all purchases included in a claim for refund shall be maintained by the claimant for inspection by the Controller or the office of the board for four years after the date of refund.