We are to look upon it as more beneficial, that many guilty persons should escape unpunished, than one innocent person should suffer. The reason is, because it’s of more importance to community, that innocence should be protected, than it is, that guilt should be punished; for guilt and crimes are so frequent in the world, that all of them cannot be punished; and many times they happen in such a manner, that it is not of much consequence to the public, whether they are punished or not.
But when innocence itself, is brought to the bar and condemned, especially to die, the subject will exclaim, it is immaterial to me, whether I behave well or ill; for virtue itself, is no security. And if such a sentiment as this, should take place in the mind of the subject, there would be an end to all security what so ever.

John Adams (1735–1826) American lawyer, Founding Father, statesman, US President (1797–1801)

Speech (1770-12-03/04), Argument for the Defense, Boston Massacre Trial
    (Source)

This is frequently (in social media) paraphrased down to:

It is more important that innocence be protected than it is that guilt be punished, for guilt and crimes are so frequent in this world that they cannot all be punished. But if innocence itself is brought to the bar and condemned, perhaps to die, then the citizen will say, “whether I do good or whether I do evil is immaterial, for innocence itself is no protection,” and if such an idea as that were to take hold in the mind of the citizen that would be the end of security whatsoever.

Adams volunteered to represent the British against murder charges, even though it was a very unpopular thing to do.

More info here: Quote Files: John Adams on Innocence, Guilt, and Punishment – The Skeptical Libertarian.