This discourse was taught by the Rebbe on Shabbat Parshat Terumah, 1983.  The command in the Torah 'Make for Me a Sanctuary' (Exodus 25:8) is commented on by the Midrash: since the text states 'for Me', this means the Sanctuary is eternal.

The Rebbe asks: in which sense is it eternal? Both Temples were destroyed..  One explanation is that there is a heavenly Temple which is never destroyed; another explanation is that there are 'small Temples', the Synagogues and Study Houses, in which the Divine dwells, and so too in the heart of each individual man and woman...