[Advaita-l] Rig mantras referred to in the Taittiriya Upanishad?

[Venkatraghavan S]

Namaste all,

In studying the bhAshya to the Ananda valli of the Taittiriya Upanishad, I
was surprised to see that the upanishad is quoting Rig mantras in several
places.

For example, the Upanishad is said to quote from the Rig Veda in defining
Brahman, the means to attain it and the benefit of attaining it "सत्यं
ज्ञानमनन्तं ब्रह्म । यो वेद निहितं गुहायां परमे व्योमन् । सोऽश्नुते सर्वान्
कामान् सह । ब्रह्मणा विपश्चितेति". Shankaracharya, in providing the
commentary of  तदेषा अभ्युक्ता says तत् तस्मिन्नेव ब्राह्मणवाक्योक्तार्थे
एषा ऋक् अभ्युक्ता आम्नाता.

I also understand that whenever the Upanishad says तदप्येष श्लोको भवति -
the sloka refers to a rig mantra.

Has anyone traced these quotations to any existing mantras in Rig Veda? If
so, please can you provide the reference.

Regards,
Venkatraghavan