This ritual is in the Book of Shadows seen in the Season 2 episode of Charmed, They're Everywhere. There was only an overview shot of it, never seen up close.
Prue also refers to it in order to protect Eric from the Collectors. She says they have to remove the knowledge from his brain forever, and that the spell is in the Book.
The entry reads thus:
To Extract Knowledge
- Crush fresh rosemary which
- has been cut with silver shears.
- Mix with the juice of a pomegranate
- and cool spring water.
- Warm the mixture,
- then rub it into your hands.
- Then place your hands on the forehead of the one
- who must release the information and recite...
- Elixir for remembrance
- Releases when warmed by fire.
- Put into safe keeping
- The knowledge I desire.
- You then must rub your hands with amethysts
- and moonstones to remove the elixir and
- to transfer the information to the stones for safe keeping.
Note[]
- A scan of the page was made when the stand in-Book, containing information from the first 3 seasons, was sold at auction.