26 CFR §48.4041-15

1. (a)Application of exemption. The taxes imposed by section 4041 do not apply in the case of a sale of any liquid by any person for the exclusive use of any State or any political subdivision thereof, the District of Columbia, or in the case of the use of any liquid by any State or any political subdivision thereof, or the District of Columbia, as a fuel in a motor vehicle, motorboat, or aircraft.
2. (b)Evidence required to establish exemption. Any vendor claiming exemption under this section shall be prepared to produce evidence that will establish the right to exemption from the tax imposed by section 4041. Generally, orders or contracts of a State or a political subdivision thereof, or the District of Columbia, when signed by an authorized officer thereof will be accepted in support of the exemption. However, in the absence of such orders or contracts, a certificate signed by such an authorized officer that the liquid sold was purchased for the exclusive use of a State or political subdivision thereof, or the District of Columbia, will be acceptable. The certificate shall be in substantially the following form: