Rules

37. Service out of Turn in Doubles
If a partner serves out of his turn, the partner who ought to have served shall serve as soon as the mistake is discovered, but all points scored, and any faults served before such discovery, shall be reckoned. If a game shall have been completed before such discovery, the order of service remains as altered.

USTA Comment: For an exception to Rule 37 see Case 3 under Rule 27.