This discourse was taught by the Rebbe on Shabbat Parsha Chukat in 1965.  It concerns the law of the Red Heifer, which is described both as the central Mitzva in the Torah and also as a Statute which cannot be understood, even by Solomon.  

The discourse explains that the statute beyond reason bonds us to G-d in a remarkable way, which then percolates through all other aspects of our relationship with the Divine and our observance of Jewish law.