Indeed the whole universe is manifestation of Self alone.
He who is devoted to the path of action, whose mind is it pure, who
has conered the self, who has subdued his senses and who realizes his
Self as the Self in all beings, though acting, is not tainted.(BG 5.7)
It moves, it is motionless. It is distant, it is near. It is within
all, it is without all this. Who sees everything in his Atman and his
Atman in everything, by that he feels no revulsion.(Isa Up 5-6)
Now that light which shines above this heaven, higher than all, higher
than everything, in the highest world, beyond which there are no other
worlds, that is the same light which is within man.(Chandogya
Upanishad 3.13.7)
This is my Soul in the innermost heart, greater than the earth,
greater than the aerial space, greater than these worlds. This Soul,
this Self of mine is that Brahman.(Chandogya Upanishad 3.14.3-4)
And when he becomes a god, as it were, or a king, as it were, or
thinks, ‘This universe is myself, who am all,’ that is his highest
state.(Brihadaranyaka Upanishad 4.3.20)
In Upadesha Sahasri verse 6, Shankaracharya says
He should first of all teach the Sruti texts establishing the oneness
of the Self with Brahman such as, " My (Chan Up 6.2.1) child, in the
beginning it (the universe) was Existence only, one alone without a
second," "Where (Chan Up 7.24.1) one sees nothing else," "All (Chan Up
7.25.2) this is but the Self", " In (Ai Up 1.1.1) the beginning all this was but the one Self " and " All (Chan Up 3.14.1) this is verily
Brahman".