The term "Shaivite" is overused nowadays. For instance, Iyer Brahmins are often called Shaivites, but they're actually followers of Adi Shankaracharya's Smartha sect (which I discuss here and here), and simply adopt Shiva as their Ishta Devata. True philosophical Shaivism is relatively rare nowadays (in contrast to philosophical Vaishnavism which is pretty common). I discussed one genuine Shaivite sect, the Lingayat sect of Basava, in my answer here. But my question is related to a more famous sect of Shaivism, known as Shaiva Siddhanta.
Just as the Sri Vaishnava sect (of which I am a member) is based on the Pancharatra Agamas and the poems of the 12 Alwars, Shaiva Siddha at is based on the Shaiva Agamas and the poems of the 63 Nayanars. In any case, the Paramokshanirasa Karika Vritti is a work by the Shaiva Siddhanta Acharya Bhatta Ramakantha which attempts to refute 20 rival sects of Hinduism. In this excerpt, Ramakantha refutes a sub-school of Kapila's Samkhya school which believed that all the Karma a person generates in this birth always bears fruit in that person's immediate next birth, not in subsequent births. Ramakantha argues that the Vedas describe so many different consequences that different actions produce, and it would be impossible for all of them to bear fruit in a single lifetime:
Sadyojyotih says... "[Actions] which have as their fruit being born as Indra and such like are taught in the Veda too; they [produce births] separately from each other." "Being born as Indra" is the meaning of "sovereignty over heaven" in [the Vedic injunction] "He who desires sovereignty over heaven should sacrifice with the Vajapeya." The sacrificial answer which has that as its fruit - [and, as indicated] by the use of "and such like" [in the verse], the Jyotishtoma and other [meritorious actions], having heaven and other [meritorious rewards] s fruit, and the killing of a Brahmin and other forbidden [actions], having hell as fruit - are [all] taught in [Vedic] scripture.... [They produce future lives separately from each other] because [actions] whose fruits are mutually contradictory cannot produce a single birth at one time.
But my question is, where do the Vedas say that doing the Vajapeya Yagna will give you sovereignty over Devaloka, i.e. make you the next Indra?
As I discuss in this answer, how the current Indra attained his position is by doing 100 Ashwamedha Yagnas, which is why he tries to stop others from doing the same. And this seems to be the standard way all the previous Indras have attained their position; concerning past Indras the Brahmanda Purana says "All of them have severally performed a hundred sacrifices." So where do the Vedas say that a single Vajapeya Yagna will give the same result?
Unfortunately the translator of Ramakanatha's work just says this:
We have not been able to locate the Vedic source of this quotation, but we note that it is quoted by other authors, for example by Jayanta Bhatta (Nyayamanjari, volume 1, p. 687,5) as shruti.
Unfortunately, I can't seem to find any English translations of Jayanta Bhatta's Nyayamanjari online, or else I could just see if the translator of that work knew the origin of the verse.
So does anyone know where in the Vedas this verse can be found?