Law is the highest reason, rooted in nature, which commands things that must be done and prohibits the opposite.

[Lex est ratio summa, insita in natura, quae iubet ea quae facienda sunt, prohibetque contraria.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

De Legibus [On the Laws], Book 1, ch. 6 (1.6) / sec. 18 [Marcus] (c. 51 BC) [tr. Zetzel (1999)]
    (Source)

Quoting "many" or "most" learned men (by which Cicero means Stoics). (Source (Latin)). Alternate translations:

"Law (say they) is the highest reason, implanted in nature, which prescribes those things which ought to be done, and forbids the contrary."
[tr. Barham (1842)]

"Law," say they, "is the highest reason implanted in nature, which prescribes those things which ought to be done, and forbids the contrary."
[tr. Barham/Yonge (1878)]

Law is the highest expression of the system of nature, which ordains what is right and forbids what is wrong.
[ed. Harbottle (1906)]

Law is the highest reason, implanted in Nature, which commends what ought to be done and forbids the opposite.
[tr. Keyes (1928)]

Law is the highest reason, inherent in nature, which enjoins what ought to be done and forbids the opposite.
[tr. Rudd (1998)]