American Bar Association Model Rules of Professional Conduct - Rule 3.7. Lawyer as Witness
(a) A lawyer shall not act as advocate at a trial in which the lawyer is likely to be a necessary witness unless:
(1) the testimony relates to an uncontested issue;
(2) the testimony relates to the nature and value of legal services rendered in the case; or
(3) disqualification of the lawyer would work substantial hardship on the client.
(b) A lawyer may act as advocate in a trial in which another lawyer in the lawyer's firm is likely to be called as a witness unless precluded from doing so by Rule 1.7 or Rule 1.9.