The Antiquities of England and Wales, Volume V
Francis Grose (1731-1792)
first published in 1785
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?