Does anyone have a source reference for Advayavajra in tantric buddhism?

This article on tantra -- [Buddhist Tantra - 5](http://www.kamakotimandali.com/blog/index.php?p=481&more=1&c=1&tb=1&pb=1) -- says > The deities of the vajrayAna are all manifestations of shUnya. > Advayavajra says in a very characteristic verse that the deities are > nothing but manifestations of shUnya and are by nature non-existent, > and whenever there is manifestation it must be shUnya in essence. The > process of evolution of deities from shUnya has four stages: the first > is the right perception of the shUnyatA or voidness, the second is its > connection with the germ syllable, the third is the conception of an > icon and the fourth is the external representation of the deity. This > statement which occurs both in the sAdhanamAlA and in advayavajra is a > very strong argument against the theory that later Buddhism was > nothing but gross idolatry. > > They are rather the voluntary manifestations of the shUnya in > accordance with the bIjamantras uttered by the worshippers, with an > appearance suitable for the function he has to discharge. Does anyone have the appropiate references to these statements?