“It is better to risk sparing a guilty person than to condemn an innocent one.”

De laudibus legum Angliae (c. 1470), reported in Bartlett's Familiar Quotations, 10th ed. (1919).

Sefer Hamitzvot [Book of the Commandments], commentary on Negative Commandment 290, as translated by Charles B. Chavel (1967); also in Defending the Human Spirit : Jewish Law's Vision for a Moral Society (2006) by Warren Goldstein, p. 269

Cases of Conscience Concerning Evil Spirits Personating Men, Witchcrafts, infallible Proofs of Guilt in such as are accused with that Crime (1692); a variant of this has become known as Blackstone's formulation, through its expression by William Blackstone in Commentaries on the Laws of England (1765 - 1769).

Maxim 113
Sentences, The Moral Sayings of Publius Syrus, a Roman Slave

Letter to Benjamin Vaughan https://books.google.de/books?id=d3UPAAAAQAAJ&pg=PA166&dq=maxim, on Blackstone's Ratio (14 March 1785).
Epistles

Attributed by Edward Seymour in 1696 during the parliamentary proceedings against John Fenwick ( "I am of the same opinion with the Roman, who, in the case of Catiline, declared, he had rather ten guilty persons should escape, than one innocent should suffer" http://books.google.com/books?id=dIM-AQAAMAAJ&pg=PA565), to which Lieutenant General Harry Mordaunt replied "The worthy member who spoke last seems to have forgot, that the Roman who made that declaration was suspected of being a conspirator himself" (Caesar was the only one who spoke in the Senate against executing Catiline's co-conspirators and was indeed suspected by some to be involved in the plot). However, the Caesar's corresponding speech as transmitted by Sallust http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Sallust/Bellum_Catilinae*.html#51 contains no such phrase, even though it appears to be somewhat similar in spirit ("Whatever befalls these prisoners will be well deserved; but you, Fathers of the Senate, are called upon to consider how your action will affect other criminals. All bad precedents have originated in cases which were good; but when the control of the government falls into the hands of men who are incompetent or bad, your new precedent is transferred from those who well deserve and merit such punishment to the undeserving and blameless.") The first person to undoubtedly utter such a dictum was in fact John Fortescue ("It is better to allow twenty criminals to mercifully avoid death than to unjustly condemn one innocent person"). It should also be noted that whether the exchange between Seymour and Mordaunt even happened is itself not clearly established http://books.google.com/books?id=IitDAAAAcAAJ&pg=PA694.
Misattributed

Source: The Space Merchants (1953), Chapter 16 (p. 186)

The Two Pioneers
1890s, Quintessence Of Ibsenism (1891; 1913)

The Clerk's Vision (1949)
Context: The world stretches out before me, the vast world of the big, the little, and the medium. Universe of kings and presidents and jailors, of mandarins and pariahs and liberators and liberated, of judges and witnesses and the condemned: stars of the first, second, third and nth magnitudes, planets, comets, bodies errant and eccentric or routine and domesticated by the laws of gravity, the subtle laws of falling, all keeping step, all turning slowly or rapidly around a void. Where they claim the central sun lies, the solar being, the hot beam made out of every human gaze, there is nothing but a hole and less than a hole: the eye of a dead fish, the giddy cavity of the eye that falls into itself and looks at itself without seeing. There is nothing with which to fill the hollow center of the whirlwind. The springs are smashed, the foundations collapsed, the visible or invisible bonds that joined one star to another, one body to another, one man to another, are nothing but a tangle of wires and thorns, a jungle of claws and teeth that twist us and chew us and spit us out and chew us again. No one hangs himself by the rope of a physical law. The equations fall tirelessly into themselves.
And in regard to the present matter, if the present matters: I do not belong to the masters. I don't wash my hands of it, but I am not a judge, nor a witness for the prosecution, nor an executioner. I do not torture, interrogate, or suffer interrogation. I do not loudly plead for leniency, nor wish to save myself or anyone else. And for all that I don't do and for all that they do to us, I neither ask forgiveness nor forgive. Their piety is as abject as their justice. Am I innocent? I'm guilty. Am I guilty? I'm innocent. (I'm innocent when I'm guilty, guilty when I'm innocent. I'm guilty when … but that is another song. Another song? It's all the same song.) Guilty innocent, innocent guilty, the fact is I quit.