Henry de Bracton, a medieval English jurist, in his book De Legibus et Consuetudinibus Angliae, defined furtum as follows:
… furtum est secundum leges contrectatio rei alienæ fraudulenta animo furandi, invito illo cuius res illa fuerit.
… theft is, according to the laws, a deceitful touching of a thing that belongs to someone else with the intent to steal, against the will of him to whom the thing belongs.
It seems unlikely that you were unaware of this rather famous definition or some derivative of it when you asked the question; if so, you might want to mention such things. Anyway, since you want existing law, as TKR wrote in his answer, you need legem latam and arrive at:
Furtum est secundum legem latam contrectatio rei alienae fraudulenta.
The text is obviuously medieval (in fact, this very sentence really begins with: Et sciendum quod furtum est …). But the particular fragment you need, as far as I can tell, has nothing particularly unclassical in it.