Rashi on Numbers 13:17/רש"י על במדבר י״ג:י״ז

1עלו זה בנגב GO UP THIS WAY SOUTHWARD — It (the South) was the worst part of the Land of Israel. He bid them spy this out first because such is the way of merchants: they show a prospective purchaser the inferior goods first, and afterwards they show the best (Midrash Tanchuma, Sh'lach 6).