There are two kinds of injustice — the one, on the part of those who inflict wrong, the other on the part of those who, when they can, do not shield from wrong those upon whom it is being inflicted. For he who, under the influence of anger or some other passion, wrongfully assaults another seems, as it were, to be laying violent hands upon a comrade; but he who does not prevent or oppose wrong, if he can, is just as guilty of wrong as if he deserted his parents or his friends or his country.

[Sed iniustitiae genera duo sunt, unum eorum, qui inferunt, alterum eorum, qui ab iis, quibus infertur, si possunt, non propulsant iniuriam. Nam qui iniuste impetum in quempiam facit aut ira aut aliqua perturbatione incitatus, is quasi manus afferre videtur socio; qui autem non defendit nec obsistit, si potest, iniuriae, tam est in vitio, quam si parentes aut amicos aut patriam deserat.]

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

De Officiis [On Duties; On Moral Duty; The Offices], Book 1, ch. 7 (1.7) / sec. 23 (44 BC) [tr. Miller (1913)]
    (Source)

Original Latin. Alternate translations:

The vice that is opposite to justice is injustice, of which there are two sorts: the first consists in the actual doing an injury to another; the second, in tamely looking on while he is injured, and not helping and defending him though we are able: for he that injuriously falls on another, whether prompted by rage or other violent passion, does as it were leap at the throat of his companion; and he that refuses to help him when injured, and to ward off the wrong if it lies in his power, is as plainly guilty of baseness and and injustice as though he had deserted his father, his friends, or his native country.
[tr. Cockman (1699)]

There are two kinds of injustice: Of the one, they are guilty who do an injury; of the other, they who, if they are able, do not defend those from injury to whom it is offered. For he who urged on by anger, or some violent passion, attempts to injure any man, lifts his hand against his brother' and he who interferes not to resist or repel the attempt, is as guilty as if he had deserted his parents, his friends, or his country.
[tr. McCartney (1798)]

But there are two kinds of injustice; the first of those who offer an injury, the second of those who have in their power to avert an injury from those to whom it is offered, and yet do it not. For if a man, prompted either by anger or any sudden perturbation, unjustly assaults another man, such a one seems as it were to lay violent hands on one's ally; and the man who does not repel or withstand the injury, if he can, is as much to blame as if he deserted the cause of his parents, his friends, or his country.
[tr. Edmonds (1865)]

Of injustice there are two kinds, -- one, that of those who inflict injury; the other, that of those who do not, if they can, repel injury from those on whom it is inflicted. Moreover, he who, moved by anger or by some disturbance of mind, makes an unjust assault on any person, is as one who lays violent hands on a casual companion; while he who does not, if he can, ward off or resist the injury offered to another, is as much in fault as if he were to desert his parents, or his friends, or his country.
[tr. Peabody (1883)]

There are two kinds of injustice: the positive injustice of the aggressor, and the negative injustice of neglecting to defend those who are wronged. To attack a man unjustly under the influence of anger or some other passion is to lay hands upon a comrade; not to defend the oppressed and shield them from injustice, is as great a crime as to desert our parents, friends, or country.
[tr. Gardiner (1899)]

There are two classifications of injustice. One part includes those who act unjustly. The other part includes men who, even if they have the power to do so, fail to protect from abuse those people against whom other men commit violence. The man who unjustly does harm to someone else, either in anger or because some other passion arounds him, acts as if he were striking a companion. But the man who does not avert an act of violence, or offer resistance if he has the power, is just as much at fault as if he betrayed his parents, or friends, or his fatherland.
[tr. Edinger (1974)]