Ariz. Rev. Stat. § 11-642
A taxpayer of the county may request, in writing, the county attorney to institute an action pursuant to section 11-641. If the county attorney fails for twenty days after the written request to institute an action, any taxpayer of the county may bring the action in the taxpayer's own name and at the taxpayer's own cost, with the same effect as if brought by the county attorney, to recover public monies from the public body of the county that ordered or caused the illegal payment. The person instituting the action shall execute a bond with two or more sureties, made payable to the defendant and conditioned that if the plaintiff fails to prosecute the action with diligence and to effect, the plaintiff will pay all damages sustained by the defendant by reason of the action and all costs incurred in the action. If the taxpayer prevails in the action, the court shall allow the taxpayer costs and a reasonable attorney fee not to exceed forty percent of the amount recovered or saved to the county.
A.R.S. § 11-642