"When the iron bird flies and horses run on wheels, the dharma will come to the land of the red faces"

"When the iron bird flies and horses run on wheels, the dharma will come to the land of the red faces" is a quote from ~700AD that is too good to check. It is also the title of a reasonably good documentary about Tibetan Buddhism in the US. Google already said that the "iron bird" is actually the name of a year. What is the surrounding context of this quote?