Knowing Brahman – Upanishads

The Self is not certainly adequately known when spoken of by an inferior person; for it is thought of variously. When taught by one who has become identified with it, there is no further cogitation with regard to it, for it is beyond argumentation, being subtler even than the atomic quantity. (Katha Upanishad, 1.2.8)

Since he is the Ear of the ear, the Mind of the mind, the Speech of the speech, the Life of the life, and the Eye of the eye, therefore intelligent people, after giving up (their self-identification with the senses) and renouncing this world, become immortal. (Kena Upanishad, 1.2)

This earth is (like) honey to all beings, and all beings are (like) honey to this earth. The shining immortal being who is in this earth, and the shining, immortal, corporeal being in the body, (these four) are but this Self. This
(Self-knowledge) is (the means of) immortality; this (underlying unity) is Brahman; this (knowledge of Brahman) is (the means of becoming) all.  (Brihadaranyaka Upanishad, 2.5.1)