“Other dogs bite their enemies, but I my friends in order to save them.”

As quoted by Anna Freud in the preface to the (1981) edition of Topsy: The Story of a Golden-Haired Chow by Princess Marie Bonaparte.
Attributed from posthumous publications

His response when "accused of treating his opponents with too much courtesy and kindness, and when it was pointed out to him that his whole duty was to destroy them", as quoted in More New Testament Words (1958) by William Barclay; either this anecdote or Lincoln's reply may have been adapted from a reply attributed to Holy Roman Emperor Sigismund:
:* Some courtiers reproached the Emperor Sigismond that, instead of destroying his conquered foes, he admitted them to favour. “Do I not,” replied the illustrious monarch, “effectually destroy my enemies, when I make them my friends?”
::* "Daily Facts" in The Family Magazine Vol. IV (1837), p. 123 http://books.google.de/books?id=aW0EAAAAQAAJ&pg=PA123&dq=destroy; also quoted as simply in "Do I not effectually destroy my enemies, in making them my friends?" in The Sociable Story-teller (1846)
Disputed

Source: The 48 Laws of Power

Source: The Coldest Girl in Coldtown

SportsCenter
Official Mix tapes, Dedication 2 (2006)

Quoted in Miguel Helft, " Google and Salesforce Join to Fight Microsoft http://www.nytimes.com/2008/04/14/technology/14google.html?_r=1&oref=slogin", New York Times (April 14, 2008).

Source: The Vicar of Wakefield (1766), Ch. 17, An Elegy on the Death of a Mad Dog, st. 8.

Essere la natura de' motti cotale, che essi come la pecora morde deono cosi mordere l'uditore, e non come 'l cane: percio che, se come cane mordesse il motto, non sarebbe motto, ma villania.
Sixth Day, Third Story
The Decameron (c. 1350)