I'm not absolutely sure. However, the most common reference I find in the midst of messy search results is mid-September 2001. It was said to Howard Kurtz of CNN by Stephen Jakes of Reuters explaining why he would call the men who flew the planes into the WTC - terrorists. The date was September 29, 2001.
http://transcripts.cnn.com/TRANSCRIPTS/ ... rs.00.html
See also this:
Now searching those terms back 8 years - freedom fighter and terrorist is not trivial. That's why I won't say for sure this is the source.
Now searching for the entire quote, I find a reference to Ramsey Clark:
http://www.iclnet.org/pub/resources/tex ... 0174a.html
This appears in "Christian Research Institute" web site and I can't find the dates of the article. Since Ramsey Clark is still alive, this article doesn't help us except to search for quotations by Ramsey Clark. I suspect the date os his quote is in 2005 but I don't have good evidence for that.
That led me to a quoteland discussion:
http://forum.quoteland.com/1/OpenTopic? ... 9561939492
And that led me to a Parliament Record for 10 March 1993
http://www.parliament.the-stationery-of ... ate-5.html
Back to search terms I tried mixing terms and fudging in ways hard to explain and got a hit in the name Uri Avnery with the almost identical line: "A terrorist is a freedom fighter in his own eyes: a freedom fighter is a terrorist in the eyes of his enemy."
But I can't place the date of the quote and Like Clarke, Avnery is still alive.
It's not that the idea is obscure or modern or unique:
"Because I do it with one small ship, I am called a terrorist. You do it with a whole fleet and are called an emperor." ~A pirate, from St. Augustine's "City of God"
Sorry about not finding the quote. Someone online said that YALE might have a reference book of quotes.