Riddle -Problem Solving

A wealthy sheik’s caravan is finally approaching an oasis after a long, hot trek across the sand. The sheik says to two lieutenants, “To that one of you whose horse gets to the oasis last, I will give this camel laden with gold.”
The caravan proceeds on; the lieutenants slow down, each waiting for the other to get ahead. By the time the rear guard of the caravan reaches the two lieutenants, they have dismounted their horses and are sitting on the sand, each waiting for the other to become so hot and thirsty that getting to the oasis can no longer be resisted.
They tell the guard the wealthy sheik’s offer and ask for help. The guard says two words to them, whereupon the lieutenants jump onto the horses and race toward the oasis.

What did the guard tell them?

Why should the lieutenants race toward the oasis if that one whose horse gets to the oasis LAST will win the gold?