As Satesat said, the first mention of the Rede in modern Wicca comes from The Ardanes or Old Laws published by Gerald Gardner in 1957 in which Gardner stated " So it is Ardane, that none shall use the Art in any way to do ill to any. how evermuch they have injured us. And for long we have obeyed this law. "Harm none" and nowtimes, many believe we exist not. So it be Ardane that this law shall still continue to help us in our plight. No one, however great an injury or injustice they receive, may use the Art in any to do ill or harm any. "
In 1959 in his book "Witchcraft Today" Gardner refers to a historical basis for the Rede in this quotation: " [Witches] are inclined to the morality of the legendary Good King Pausol, "Do what you like so long as you harm no one". But they believe a certain law to be important, "You must not use magic for anything which will cause harm to anyone, and if, to prevent a greater wrong being done, you must discommode someone, you must do it only in a way which will abate the harm ."
In 1964, Doreen Valiente, who was Gardner's first High Priestess addressed a group of Wtiches in Britain stating, "Demanding tolerance between covens as well as toward the outside world, Doreen spoke the Anglo-Saxon witch formula called the Wiccan Rede or wise teaching: "Eight words the Wiccan Rede fulfil, An' it harm none, do what ye will."
Valiente's earlier book, An ABC of Witchcraft Past & Present , which was first published in 1973, had no specific entry for the Rede, despite introducing it in her 1964 speech. Chances are it had not yet "taken hold" in the early Wiccan "community" that was still largely segregated and coven-centric by 1973, and thus was not yet something established enough to be included in an encyclopedia of witchcraft. However in the entry on Basic Beliefs of Witches, the a variation of the Rede was mentioned as part of the discourse on the Witches' ethics:
Witches do not believe that true morality consists of observing a list of thou-shalt-nots. Their morality can be summed up in one sentence, "Do what you will, so long as it harms none." This does not mean, however, that witches are pacifists. They say that to allow wrong to flourish unchecked is not 'harming none'. On the contrary, it is harming everybody.
This is a perfect example of the perception of Wiccan ethics prior to the 1980's. Witches were not the epitome of "light and love" but rather real people who dealt with real situations, not afraid to get their "hands dirty" when necessary. Witches had a respect for life that was balanced with both its nurturing aspects and the harsh reality of the fight for survival. The Rede was a summary or point of reference, but not a complete ethical system in itself.
Gwen Thompson's version of the Rede was published in " Green Egg " magazine in 1974 and alledged to have come to her from her mother. Interestingly enough, Gwen, who was the founder of the New England Coven of Traditionalist Witches (NECTW) was not a Wiccan, nor was her coven a Wiccan group. Yet it is Thompson's version of the Rede which most Wiccans quote when they are referring to it. However, it seems very likely that Lady Gwen "borrowed" her version of the Rede from the writings of Gardner and Valiente.
In 1978 Doreen Valiente, in her book " Witchcraft for Tomorrow ", again mentions the Rede which she states was comprised of the words " Eight Words the Wiccan Rede fulfil: An it harm none, do what ye will. " She then, in the same book writes a far longer Witches' Creed which ends with the words:
" An Do What You Will be the challenge,
So be it in Love that harms none,
For this is the only commandment,
By Magick of old, be it done"
I would suggest the following websites as good references on the history and composition of the Rede:
I will point out here that the Rede is not a law. Rede is an Old English word meaning "advice" or "counsel". Nor does the Rede say that one cannot do anything that causes harm. In the archaic language of the Rede the word "An" should be read as "If". So what the Rede says is that you are free to do any action if it will not cause harm. But the Rede is silent on those actions which might cause harm, leaving that up to the situation and the ethics of the individual Wiccan.
I will also make mention that the Rede is something that is Wiccan in origin and practice. It does not have any impact on any other form of Pagan religion nor any other form of magical practice. That doesn't mean that other Pagan religions have no ethical codes at all, they most certainly do. But those other codes are not the Wiccan Rede.
As an aside, the version of the Rede quoted in the first post in this thread is attributed to The Servants of the Elder Gods in 1992. It can be found at http://www.sacred-texts.com/bos/bos296.htm