In his chapter on Lurianic Kabbalah in Major Trends in Jewish Mysticism, Scholem refers to a Midrashic source that Luria creatively re-reads to ground his novel concept of tzimtzum. The bibliography tells me that the Midrash in question is the discussion of Exodus 25.10 in Exodus Rabbah. I think I can search for the index of a source-text on Sefaria, though. Does anyone know where exactly this verse comes up in Exodus Rabbah?
I would think it’d be pretty well known given its famous Kabbalistic re-imagining. Scholem’s synopsis, for reference, is: “The Midrash occasionally refers to God as having concentrated his divine presence, in the holiest of holies, at the place of the Cherubim, as though His whole power were concentrated and contracted in a single point” (260).