It’s throughout his work, from the beginning to the end. From BGE:

In the past, every elevation of the type ‘human being’ was achieved by an aristocratic society—and this will always be the case: by a society that believes in a great ladder of hierarchy and value differentiation between people and that requires slavery in one sense or another. Without the grand feeling of distance that grows from inveterate class differences, from the ruling caste’s constant view downwards onto its underlings and tools, and from its equally constant practice in obeying and commanding, in holding down and holding at arm’s length—without this grand attitude, that other, more mysterious attitude could never exist, that longing for ever greater distances within the soul itself, the development of ever higher, rarer, more far-flung, extensive, spacious inner states, in short, the elevation of the type ‘human being’, the continual ‘self-overcoming of the human’, to use a moral formula in a supra-moral sense.