That's a really good question. That's what I've always called it, so I assumed it was in the bible, but I can't find it except in the LXX. But it is an accurate translation of both the Hebrew and Greek (άγια αγίων), I just don't know where I got it from.
And thou shalt put the veil on the posts, and thou shalt carry in thither within the veil the ark of the testimony; and the veil shall make a separation for you between the holy and the holy of holies. Exodus 26:33 (LXX)