When was the Sign of the Cross first documented?

Answer:
"The first certain evidence we have of the use of this sign is from the ecclesiastical writer Tertullian (230 AD) who tells us that candidates for baptism are marked with a sign of the cross on their foreheads during the course of their catechumenate (formation)."
--  Colin B. Donovan, STL (http://www.ewtn.com/expert/answers/sign_of_the_cross.htm)

I honestly didn't know the answer to this. Of course the Sign of the Cross points to the trinity of three persons (Father, Son and Holy Spirit) in one God. And the cross is the symbol of our salvation. In looking around on the internet I found multiple sources with this answer. Interesting to note that the tradition of signing with a cross goes back so far to the early days of Christianity.

Let me know if you have any other interesting facts about the history of this symbolic prayer.