That Thou Art (Tat Tvam Asi) - Mahavakyas from Upanishads

hinduwiki.com — That Thou Art (tat tvam asi) is a grand pronouncement (mahavakya) that occurs in Chandogya Upanishad. This is great dictum on which Sankaracharya wrote an extensive commentary in the his 'Vakya Vritti'. This great dictum expounds the unity of individual and Universal and denies the existance of a separate individuality apart from Brahman.