After a survey of all our civil wars, with a wealth of detail you urged the Senate, while they feared the warnings of the civil wars within their memory, to draw the inference that, as the earlier combatants had shown a ruthlessness hitherto quite unprecedented in the Republic, so whosoever should subsequently succeed in crushing the Republic by force of arms would display a tyranny far more intolerable. For men assume that what is done by precedent is also done by right; but they add to that precedent and contribute to it something, nay rather, a great deal of their own.

[Accuratissime monuisti senatum conlectis omnibus bellis civilibus, ut et illa timerent, quae meminissent, et scirent, cum superiores nullo tali exemplo antea in re publica cognito tam crudeles fuissent, quicumque postea rem publicam oppressisset armis, multo intolerabiliorem futurum. nam, quod exemplo fit, id etiam iure fieri putant, ipsi aliquid atque adeo multa addunt et adferunt de suo.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher
Epistulae ad Familiares [Letters to Friends], Book 4, Letter 3, sec. 1 (4.3.1), to Servius Sulpitas Rufus (46 BC ) [tr. Williams (Loeb) (1928)]
    (Source)

(Source (Latin)). Alternate translation:

You admonished the Senate, to take heed, by the calamities that were past: and to thinke how much more intollerable those would be, which oppressed the Country in these times, seeing they had beene so cruell, who oppressed it heretofore, having thereof no former president or example: in that men, doe usually thinke, that they may in all reason doe, what they have example for; nay, and to doe worse, then their president allowes.
[tr. Webbe (1620)]

You prudently endeavoured to awaken our fears, by enumerating those civil wars that had happened within our own memories. And if the authors of these, you told the house, unsupported by a single example of the same kind to give a colour to their conduct, had exercised such dreadful cruelties; whoever in future times should successfully turn his arms against the republic, would most assuredly prove a much more intolerable tyrant. For they that act by precedent, you observed, generally think they act by right: and in cases of this nature seldom fail of iimproving on their model.
[tr. Melmoth (1753), 9.1; (1814 ed) # 106]

You warned the senate in the most impressive terms, both to fear those they remembered, and to feel assured, since the last generation had been so cruel -- to an extent up to that time unprecedented in the Republic -- that whoever thenceforth overpowered the Republic by arms would be much more difficult to endure. For what is done on a precedent, they Consider as even legally justifiable: but they add and Contribute something, or rather a great deal, of their own to it.
[tr. Shuckburgh (1899), # 492]

Using the shorter, more commonly quoted Quod exemplo fit, id etiam jure fieri putant, there are two common translations used in various sources:

Men think they may justly do that for which they have a precedent.
[E.g. (1882)]

What is shown by example, men think they may justly do.
[E.g. (1937)]