Here is the premise of the argument given by Shaivas and Vaishnavas to show that their deities are the supreme Brahman. Quoting and slightly paraphrasing from Nadadur Ammal's Para Tattva Nirnaya:
To explain, it is understood that only the cause of the universe is the supreme reality, as the following [Taittiriya Upanishad] verse says: 'That from which all beings are born, that by which they live, and that into which they enter at their death, try to know that: that is Brahman.'
This is the definition of Brahman according to the second sutra of the Brahma Sutras:
When it is questioned, 'Which is that cause?' The Chhandogya Upanishad reveals that the cause of the Universe is denoted by the word 'Sat' in the verse, 'This universe was at first, my dear, only Sat.' The cause of the universe is then referred to as Brahman in the Vajasaneyaka verse, 'Brahman indeed this was at first.' By the rule of sarvasakha-pratyaya there can only be one entity as the cause designated by all these terms.
The word 'Sat' is generic term meaning existent, and it will denote any object big or small. The word 'Brahman' meaning 'big' is a more specific term. According to chaga-pashu-nyaya, the generic term is specified by the more specific term when the context [in this case origination] is the same, the entity designated 'Sat' is specified by the term 'Brahman.' Therefore, the cause is not only 'existent' but also 'big'.
Then, another more specific designation for the cause is particularized in the Aitareya Upanishad by the word 'Atman' in the verse, 'This existed in the beginning as the Atman alone." This rules out insentient beings as the cause, so that the cause is now only identified as a sentient being. So now, the cause is 'Sat', 'Brahman', and 'Atma'.
The word 'Atma' is common to all sentient beings, and now the question arises, which sentient being is the cause?
At this point, the Shaivas cite the following Shvetashvatara Upanishad verse,
Shiva alone existed
Followers of Brahma cite this verse,
Hiranyagarbha existed at first
While Vaishnavas cite this Mahopanishad verse,
Narayana alone existed, not Brahma, not Ishana [Shiva]
Is there any verse in the Rig Veda Samhita that specifically says something like this for Indra? Like "In the beginning, Indra alone existed", "In the beginning, Maghavan alone existed"?