Taittiriya Upanishad.

 He energised conscious-force (in the austerity of thought) and

came to the knowledge that Matter is the Brahman. For from
Matter all existences are born; born, by Matter they increase
and enter into Matter in their passing hence. Then he went to
Varuna, his father, and said, “Lord, teach me of the Brahman.”
But he said to him: “Energise (again) the conscious-energy in
thee; for the Energy is Brahman.”