I can’t really remember. But I read somewhere where he says to keep telling yourself “Aham Brahmasmi”....and other mahavakyas until it becomes the living reality
There are many such quotes of Swami Vivekananda which have that purport. Here are only a few of those:
You are the incarnations of the Almighty, Omnipresent, divine principle (VIII.37)
You are all God. (II.237)
You are the Self, the God of the universe. (II.236)
You are the almighty God of the universe. (II.236)
We are God Himself though we have forgotten our own nature in thinking of ourselves as little men. (III.160)
You are God and so am I; who obeys whom? (II.320)
You are the Self, and that must be realized. (IV. 245)
You are free, free, free! (VII.61)
Therefore know that thou art He; thou art the God of this universe, Tat Tvam Asi (that thou art). (II.236)
Thou art Reality. (VIII.106)
You are all infinite. (II.470)
You are the Infinite. (II.462)
Here, the II.462 etc are indicating the locations of the corresponding sayings in his Complete Works.
I have collected all these sayings from the book "Pearls of Wisdom-Swami Vivekananda"; published by RKMIC, Calcutta.