M. Tullius Cicero Pro L. Murena 8

[8] Nor does it follow, because I stood by you when you were a candidate for the consulship, that on that account I ought now to be an assistant to you in the same way, when you are attacking Murena himself. And this it not only not praiseworthy,—it is not even allowable, that we may not defend even those who are most entirely strangers to us when our friends accuse them.
  4 But in truth, there is, O judges, between Murena and myself an ancient and great friendship, which shall not be overwhelmed in a capital trial by Servius Sulpicius, merely because it was overcome by superior considerations when he was contesting an honorable office with that same person. And if this cause had not existed, yet the dignity of the man, and the honourable nature of that office which he has obtained, would have branded me with the deepest reproach of pride and cruelty, if in so great a danger I had repudiated the cause of a man so distinguished by his own virtues and by the honours paid him by the Roman people. For it is not now in my power,—it is not possible, for me to shrink from devoting my labour to alleviate the dangers of others. For when such rewards have been given me for this diligence of mine, such as before now have never been given to any one, to abandon those labours by which I have earned them, as soon as I have received them, would be the act of a crafty and ungrateful man.