California Code, Public Contract Code - PCC § 21286

All proposals or bids offered shall be accompanied by a cashier's or certified check payable to the district, for an amount which shall not be less than 10 percent of the aggregate of the proposal, or by a bond for that amount and so payable, signed by the bidder and two sureties, who shall justify, in double that amount, before any officer competent to administer an oath, and over and above all statutory exemptions, or by a corporate surety bond, to the satisfaction and approval of the district.