This is a fragment from chapter 6.8-6.16 of the Chandogya Upanishad, see: AS Gupta (1962), The Meanings of "That Thou Art", Philosophy East and West, Vol. 12, No. 2, pages 125-134; for its different meanings, see Joel Brereton (1986), Tat Tvam Asi in Context, Zeitschrift der deutschen morgenlandischen Gesellschaft, Vol 136, pages 98-109, and Robert Hume, Chandogya Upanishad, The Thirteen Principal Upanishads, Oxford University Press, pages 245-246