The Gauls and Their Demons
Augustine of Hippo

Liber XV.
XXIII. Et quoniam creberrima fama est multique se expertos uel ab eis, qui experti essent, de quorum fide dubitandum non esset, audisse confirmant, Siluanos et Panes, quos uulgo incubos uocant, inprobos saepe extitisse mulieribus et earum appetisse ac peregisse concubitum; et quosdam daemones, quos Dusios Galli nuncupant, adsidue hanc inmunditiam et temptare et efficere, plures talesque adseuerant, ut hoc negare inpudentiae uideatur[.]

Book 15.
Chapter 23. There is, too, a very general rumor, which many have verified by their own experience, or which trustworthy persons who have heard the experience of others corroborate, that sylvans and fauns, who are commonly called "incubi," had often made wicked assaults upon women, and satisfied their lust upon them; and that certain devils, called Duses by the Gauls, are constantly attempting and effecting this impurity is so generally affirmed, that it were impudent to deny it.

SOURCES

Latin:
Augustine. De Civitate Dei. URL: http://www.thelatinlibrary.com/augustine/civ15.shtml

English:
Augustine of Hippo. The City of God. trans. Marcus Dods. Nicene and Post-Nicene Fathers, First Series, Vol. 2. Buffalo, NY: Christian Literature Publishing Co., 1887. URL: http://en.wikisource.org/wiki/The_City_of_God/Book_XV/Chapter_23