“It is more dangerous that even a guilty person should be punished without the forms of law than that he should escape.”

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

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

Victor Frankenstein of Justine Moritz in Ch. 8
Frankenstein (1818)

In Adams' Argument for the Defense in the case of Rex v. Wemms: Suffolk Superior Court, Boston, 3-4 December, 1770; source "The Adams Papers", http://rotunda.upress.virginia.edu/
1770s

Endorsement of a letter relating to the Whiskey Ring (29 July 1875).
1870s

Book XXXIV, sec. 4
History of Rome

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).

On Crimes and Punishments (1764)

Book IV, ch. 27.
Commentaries on the Laws of England (1765–1769)