Tex. R. Civ. P. 185
When any action or defense is founded upon an open account or other claim for goods, wares and merchandise, including any claim for a liquidated money demand based upon written contract or founded on business dealings between the parties, or is for personal service rendered, or labor done or labor or materials furnished, on which a systematic record has been kept, and is supported by the affidavit of the party, his agent or attorney taken before some officer authorized to administer oaths, to the effect that such claim is, within the knowledge of affiant, just and true, that it is due, and that all just and lawful offsets, payments and credits have been allowed, the same shall be taken as prima facie evidence thereof, unless the party resisting such claim shall file a written denial, under oath. A party resisting such a sworn claim shall comply with the rules of pleading as are required in any other kind of suit, provided, however, that if he does not timely file a written denial, under oath, he shall not be permitted to deny the claim, or any item therein, as the case may be. No particularization or description of the nature of the component parts of the account or claim is necessary unless the trial court sustains special exceptions to the pleadings.
Tex. R. Civ. P. 185