As I discuss in the this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa which summarizes and systematizes the philosophical teachings of the Upanishads. You can read the Brahma Sutras here. In any case, Adhyaya 3 Pada 3 of the Brahma Sutras describes the Brahma Vidyas, 32 lessons found in the various Upanishads which can each lead you to Brahman if you meditate on them. You can see the full list of 32 Vidyas here.
One of these Brahma Vidyas is known as the Paryanka Vidyas, and it's described in the first chapter of the Kaushitaki Brahmana. Part of the chapter describes soul seeking Moksha and his conversation with Brahman (although Advaitins would say that this is merely Saguna Brahman or Ishwara, AKA Sriman Narayana.) Here is part of that conversation:
Brahman says to him: 'Who am I?' He shall answer: 'That which is, the true' (Sat-tyam).
Brahman asks: 'What is the true?' He says to him: 'What is different from the gods and from the senses (prâna) that is Sat, but the gods and the senses are Tyam. Therefore by that name Sattya (true) is called all this whatever there is. All this thou art.'
This is also declared by a verse: 'This great Rishi, whose belly is the Yagus, the head the Sâman, the form the Rik, is to be known as being imperishable, as being Brahman.'
My question is, where is this verse from? Is it a verse from the Vedas, or somewhere else?
Note that I'm not asking for the meaning of the verse; that's presumably discussed in the various commentaries on the Upanisahad. I'm just looking for the origins of the verse.