The White Witch of Waverly, from Grose's Antiquities (1785)

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The Antiquities of England and Wales, Volume V
Francis Grose (1731-1792)
http://books.google.com/books?id=zZIFAAAAQAAJ
first published in 1785

"The place derives its name from a popular story, which make it formerly the residence of a white witch, called Mother Ludlam, or Ludlow; not one of those malevolent beings mentioned in the Daemonlogie, a repetition of whose pranks, as chronicled by Glanvil, Baxter, and Cotton Mather, erects the hair, and closes the circle of the listening rustics round the village fire. This old lady neither killed hogs, rode on broom-staves, nor made children vomit nails and crooked pins; crimes for which many an old woman has been sentenced to death by judges, who, however they may be vilified in this skeptical age, thereby certainly cleared themselves from the imputation of being wizards or conjurors. On the contrary, Mother Ludlam, instead of injuring, when properly invoked, kindly assisted her poor neighbors in their necessities..."

The above passage is from pages 111-112 of  the edition linked to above (at googlebooks). If you have another edition just be sure it is volume V, and look for the section on Surrey.

There are many later references to "Mother Ludlam", "The White Witch of Waverly", etc. But Grose is the earliest source I have found to use the phrase "white witch". In fact, Grose is so far the earliest source I have found to use this phrase after the end of the period of the Witch Hunts. Indeed, Grose is a full 36 years earlier than Walter Scott's Kenilworth, which had previously been the earliest post-Witch-Hunt reference to "white Witches" that I had found.

Note to Ronald Hutton, Owen Davies, and Jacqueline Simpson: hey, you guys are supposed to be the pros. If I can find this stuff, why can't you?