Everybody knows that the first law of history is not daring to say anything false; that the second is daring to say everything that is true; that there should be no suggestion of partiality, none of animosity when you write.

[Nam quis nescit primam esse historiae legem, ne quid falsi dicere audeat? Deinde ne quid veri non audeat? Ne quae suspicio gratiae sit in scribendo? Ne quae simultatis?]

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

De Oratore [On the Orator, On Oratory], Book 2, ch. 15 (2.15) / sec. 62 (55 BC) [tr. May/Wisse (2001)]
    (Source)

(Source (Latin)). Alternate translations:

For, is there a Man ignorant, that the first Rule of History is, that an Historian shall not dare to advance a Falsity; the next, that there is no Truth but what he shall dare to tell? That in Writing, he shall be free of all Prepossession; of all Pique?
[tr. Guthrie (1755)]

For, is there a man ignorant that the first rule of history is that an historian shall not dare to advance a falsehood; the next, that there no truth but what he shall dare to tell? That the writer should be actuated neither by favour, or by prejudice?
[Source (1808)]

For who is ignorant that it is the first law in writing history, that the historian must not dare to tell any falsehood, and the next, that he must be bold enough to tell the whole truth? Also, that there must be no suspicion of partiality in his writings, or of personal animosity?
[tr. Watson (1860)]

Who need be informed that the first law of history is, to have the honesty to state no falsehood, the next, the courage to suppress no truth, and to avoid all suspicion of undue bias or personal animosity?
[tr. Calvert (1870)]

Who does not recognise that the first law of history is that we shall never dare to say what is false; the second that we shall never fear to say what is true; that everything we write shall be free from any suspicion of favoritism or flattery?
[ed. Harbottle (1906)]

For who does not know history's first law to be that an author must not dare to tell anything but the truth ? And its second that he must make bold to tell the whole truth? That there must be no suggestion of partiality anywhere in his writings? Nor of malice?
[tr. Sutton/Rackham (1940)]

The first law for the historian is that he shall never dare utter an untruth. The second is that he shall suppress nothing that is true. Moreover, there shall be no suspicion of partiality in his writing, or of malice.
[Bartlett's]