Rashi on Deuteronomy 32:37/רש"י על דברים ל״ב:ל״ז

1ואמר AND HE, the Omnipresent, WILL SAY with regard to them אי אלהימו WHERE ARE THEIR GODS which they served.

2צור חסיו בו THE ROCK IN WHOM THEY TRUSTED — i.e. the rock beneath which they sought shelter (more lit., with which they covered themselves) from the heat and the cold; that is to say, “in which they trusted” that it would protect them against evil.