The first chapter of the Katha Upanishad, in the recensions commented on by Adi Shankaracharya and Madhvacharya, has 29 verses. But the recension of the Katha Upanishad commented on by the Sri Vaishnava Acharya Ranga Ramanuja contains 30 verses in chapter 1. The additional verse occurs between the verses numbered 18 and 19 in the other recensions, and it says this:
yo vāpyetāṃ brahmajajñātmabhūtāṃ citiṃ viditvā cinute nāciketam ।
sa eva bhūtvā ahmajajñātmabhūtaḥ karoti tadyena punarna jāyate ॥
Whoever knowing this piling up which is the self-born of Brahman and sentient constructs Nachiketa (fire-altar), that very person becoming the born of Brahman and sentient, does that by which (he) is not born again.
Ranga Ramanuja, in his commentary on this verse, says this:
This Mantra is not seen in some texts and was not commented on by some. Since, however, this is commented on by such most reliable (authorities) as Vyasarya and others, the doubt as to its being an interpolation is not proper.
I’m interested in the part in bold. Vyasarya refers to the 13th century Sri Vaishnava Acharya Sudarshana Suri, because he was the grandson of Vedavyasa Bhattar, son of Kurathalwan and brother of Parashara Bhattar. And indeed, Sudarshana Suri quotes this verse in his Shruta Prakshika, his subcommentary on Ramanujacharya’s Sri Bhashya:
"yo vā etāṃ brahmajajñātmabhūtāṃ citiṃ viditvā cinute nāciketam । sa eva bhūtvā brahmajajñātmabhūtaḥ karoti tadyena punarna jāyate" iti vākyena brahmātmakacetanadṛṣṭyā vā tādṛśadevatādhiṣṭhitatvena vā'nusandhānapūrvakamagni cinvānasyā apunarbhavahetubhūta-brahmopāsanadvārā mokṣaprāptirhyuktā
In [the verse quoted] it is said that focusing on the Brahman being the indweller (of the Nachiketa fire altar) when performing the piling of the alter leads to the meditation of himself having Brahman as his self and ultimately leads to Moksha.
Now Ranga Ramanuja says it’s quoted by “Vyasarya and others”. So my question is, who else quotes this verse? What is the earliest reference to this verse?