Let us now consider what follows. “They have also,” says he, “a precept to this effect, that we ought not to avenge ourselves on one who injures us, or, as he expresses it, 'Whosoever shall strike you on the one cheek, turn to him the other also.' This is an ancient saying, which had been admirably expressed long before, and which they have only reported in a coarser way. For Plato introduces Socrates conversing with Crito as follows: 'Must we never do injustice to any?' 'Certainly not.' 'And since we must never do injustice, must we not return injustice for an injustice that has been done to us, as most people think?' 'It seems to me that we should not.' 'But tell me, Crito, may we do evil to any one or not?' 'Certainly not, O Socrates.' 'Well, is it just, as is commonly said, for one who has suffered wrong to do wrong in return, or is it unjust?' 'It is unjust.
Yes; for to do harm to a man is the same as to do him injustice.' 'You speak truly. We must then not do injustice in return for injustice, nor must we do evil to any one, whatever evil we may have suffered from him.' Thus Plato speaks; and he adds, 'Consider, then, whether you are at one with me, and whether, starting from this principle, we may not come to the conclusion that it is never right to do injustice, even in return for an injustice which has been received; or whether, on the other hand, you differ from me, and do not admit the principle from which we started.
That has always been my opinion, and is so still.' Such are the sentiments of Plato, and indeed they were held by divine men before his time. But let this suffice as one example of the way in which this and other truths have been borrowed and corrupted. Any one who wishes can easily by searching find more of them.”
Source: Against Celsus (New Advent)