Сhange the terms

A king, in order to select which one of his two sons will inherit his fortune, decides to have his sons compete in a horse race.
This is an unusual horse race, however, as the king specifies that the one with the slower horse will inherit the kingdom.

So, the two brothers proceed very slowly, and a few days later have made very little progress. They come across a wise man and ask him for advice.
The wise man speaks two words, after which the brothers jump on the horses and race as fast as they can.

What did the wise man say?