California Evidence Code Section 403
(a) The proponent of the proffered evidence has the burden of
producing evidence as to the existence of the preliminary fact, and
the proffered evidence is inadmissible unless the court finds that
there is evidence sufficient to sustain a finding of the existence of
the preliminary fact, when:
(1) The relevance of the proffered evidence depends on the
existence of the preliminary fact;
(2) The preliminary fact is the personal knowledge of a witness
concerning the subject matter of his testimony;
(3) The preliminary fact is the authenticity of a writing; or
(4) The proffered evidence is of a statement or other conduct of a
particular person and the preliminary fact is whether that person
made the statement or so conducted himself.
(b) Subject to Section 702, the court may admit conditionally the
proffered evidence under this section, subject to evidence of the
preliminary fact being supplied later in the course of the trial.
(c) If the court admits the proffered evidence under this section,
(1) May, and on request shall, instruct the jury to determine
whether the preliminary fact exists and to disregard the proffered
evidence unless the jury finds that the preliminary fact does exist.
(2) Shall instruct the jury to disregard the proffered evidence if
the court subsequently determines that a jury could not reasonably
find that the preliminary fact exists.