Where does the Catholic tradition that Mary did not have pain giving birth to Jesus come from?

Where does the Catholic tradition that Mary's birth of Jesus was painless come from? My wife just got home from a non-denominational study about Mary and she said she thought everybody knew this, but apparently no one knew about it - especially the Protestants! In any event, what I'm looking for is the origin of the quote that Jesus' birth was "like light passing through glass" and whether that quote is wholly indicative of a painless birth or if it could have been painless in another way? I understand that it's a little t tradition in the Church, not a dogma or anything. But it's definitely a strong tradition; although it's one that could use a little preaching on from what I can tell.