Cal. Rev. & Tax. Code § 6386

There are exempted from the computation of the amount of the sales tax the gross receipts from the sale in this state of tangible personal property to a holder of a valid seller’s permit issued under Section 6067 when the property is used by the purchaser outside of this state in his performance of a contract to improve real property and, as a result of such use, is incorporated into and becomes a part of real property located outside of this state. This exemption shall apply only if the purchaser certifies in writing to the seller, in such form as the board may prescribe, that the property will be used in a manner and for a purpose herein specified.