The Emerald Tablet

The Emerald Tablet is a brief alchemical text that was included in some of the earliest Arabic treatises on alchmey, such as the Book of the Secret of Creation and the Book of the Secret of Secrets. These texts date to the eighth and nineth centuries. The text (above) was supposed to have been written by the legendary sage Hermes Trismegistus. Wikipedia has the whole text in translation; the most famous line is "the uppermost is from the lowermost and the lowermost is from the uppermost."

The best thing about this text it is the story of its discovery, which was included in both of those key Arabic manuscripts. The text was supposed to come from an actual emerald tablet that was found in the hand of a corpse lying on the floor of a tomb, in front of a statue of Hermes Trismegistus. In one manuscript this was the tomb of Hermes himself.

Which is the most Dungeons & Dragons story I know from the whole medieval age.