Bonds payable from the utility bond fund shall not be an obligation of the district and they shall state on their face that they are payable solely from a special fund derived from a certain fixed proportion (naming it) of the gross income derived by the district from the sale or lease of water or from other service, as the case may be, and such fixed proportion of such gross income shall be irrevocably devoted to the payment of such bonds with interest until the same are fully paid.