Generally, Vaishnavas follow the Pancharatra Agamas, Shaivites follow the Shaiva Agamas, and Shaktas follow the Shaiva Agamas. But long ago, there was a group of people in Kashmir who followed the Shakta Agamas but adopted Shiva as their Ishta Devata. That group evolved into what we now call Kashmir Shaivism. Because it has its roots in Shakta Agamas, it subscribes to a monistic philosophy, as opposed to philosophic Shaivite sects like Shaiva Siddhanta which are more dualistic in their philosophy.
Now one important Kashmiri Shaivite text is the Virupaksha Panchashika, which takes the form of a dialogue between Indra and Shiva's incarnation Virupaksha. In this excerpt from the Virupaksha Panchashika, Virupaksha tells Indra that other texts agree with what he has been saying:
Indeed, in the writings of the sages Shuka, Vamadeva, Krishna, Dadhichi, and Vainya, identity with the universe is explained to born of the yoga of identification with the elements.
Now most of these are recognizable: the work of Vamadeva probably refers to the two Vedic hymns I discuss here, the work of Krishna probably refers to the Bhagavad Gita, and the work of Dadhichi probably refers to the Isha Upanishad (see this answer). But I'm interested in the last name mentioned, Vainya. That's a reference to Vishnu's incarnation Prithu, who was the son of the evil ruler Vena.
As described in numerous scriptures including this chapter of the Vishnu Purana, Vena put a ban on Yagnas, which led to Bhumidevi goddess of the Earth deciding to stop producing crops. So the Rishis gathered together and killed Vena, and then from his body they produced a new heir to the throne, namely Vishnu's incarnation Prithu. Prithu chased Bhumidevi, who took the form of a cow, and convinced her to start producing crops again by promising to be her protector. That is why Bhumidevi is known as Prithvi. Prithu was also the first human ruler to be given the titles Raja or king and Chakravarthi or emperor.
But my question is, what work of Prithu is Virupaksha referring to, in which "identity with the universe is explained to born of the yoga of identification. with the elements"? The story of Prithu is described in the Mahabharata and many Puranas, including the Vishnu Purana chapter I linked to above. And he's mentioned in the Brahmanas of the Vedas, for instance in this chapter of the Shatapatha Brahmana of the Yajur Veda. But none of those would qualify as works of Prithu.
The only thing I can think of that might qualify as a work of Prithu is Rig Veda Book 10 Hymn 148 which was heard during Tapasya by Prithu as you can see in my answer here. But that's just a hymn to Indra, not something about being one with the Universe. So does anyone know what work the Virupaksha Panchashika is referring to?