Cal. Civ. Proc. Code § 1297.72

An arbitration agreement shall be in writing. An agreement is in writing if it is contained in a document signed by the parties or in an exchange of letters, telex, telegrams, or other means of telecommunication which provide a record of this agreement, or in an exchange of statements of claim and defense in which the existence of an agreement is alleged by one party and not denied by another. The reference in a contract to a document containing an arbitration clause constitutes an arbitration agreement provided that the contract is in writing and the reference is such as to make that clause part of the contract.