During British rule, the town of Tirupati including its world-renowned Tirumala Venkateshwara Temple came to be controlled by the East India Company, starting in 1801. And the first East India Company official to control the area, a Collector named Stratton, was very interested in the temple, since it's one of the holiest sites in Hinduism, so he sent a bunch of questions to the temple officials asking all about the temple's history, traditions, stories, etc. The record of his questions and their answers is known as the "Sawal-E-Jawab".
In any case, this excerpt from the Sawal-E-Jawab claims that Venkateshwara (the Vishnu deity in Tirupati, AKA Srinivasa or Balaji) is alluded to in the Taittirya Upanishad of the Yajur Veda:
It is said in the Upanishads that [Venkateshwara] is the all-pervading Vishnu present in Vaikunta, on the earth and everywhere. In the Taittiryopanishad, in the fifth Parva called "Narayanam", it is stated that the Lord shines at the farther end of the water, in the middle of this earthly region, and that he makes the other elements shine through his own light. This is quoted from the commentary of Vidyaranya on the said Upanishad.
The reason that "the Lord shines at the farther end of the water" seems like a reference to Venkateshwara is that the Tirumala Venkateshwara Temple is located at the Southern end of the Swami Pushkarini lake.
But my question is, where exactly is this statement in the Taittirya Upanishad? It says it is in the "fifth Parva", but as far as I can tell there's no such thing. You can read the Taittirya Upanishad here; it's only divided into three Vallis, and each Valli is divided into a bunch of Anuvakas. And I tried searching through the work but I couldn't find any such quote. Could the statement be from the Mahanarayana Upanishad instead? The Taittirya Upanishad constitutes the seventh, eighth, and ninth Vallis of the Taittirya Aranyaka of the Yajur Veda. But the tenth Valli of the Taittirya Aranyaka is called the Mahanarayana Upanishad, and the Sawal-E-Jawab does say that the quote is from something called the "Narayanam".
In any case, the Sawal-E-Jawab isn't even claiming to take the statement directly from the Taittirya Upanishad, but rather from a commentary on the Upanishad by Vidyaranya, the brother of Sayana (who wrote a definitive commentary on the Vedas). Does anyone know if Vidyaranya's commentary discusses the quote about being on the farther end of the water?
(P.S. On a side note, some people may be wondering how the Vedas could possibly mention Venkateshwara, when the Vedas were around long before Rama and Krishna, and Venkateshwara came to the earth after Krishna. But when people talk about the Vedas being of unfathomable age, they're talking about the the mantras found in the Samhitas of the Vedas. Those mantras were heard directly from the gods by sages over countless generations, but they were compiled into the Vedic Samhitas by Vyasa at the end of the Dwapara Yuga. So presumably the Brahmanas, Aranyakas, and Upanishads were only added to the Samhitas some time after the beginning of the Kali Yuga. So since the Venkateshwara story took place shortly after the death of Krishna and the start of the Kali Yuga, there would have been enough time for information about him to be included in Upanishads.)