Hic salta: why

Proverb

hic Rhodus, hic salta:  (politics) Prove what you can do, here and now.

Aesop's Fables, translated by Laura Gibbs (2002)

209. THE BOASTFUL ATHLETE

There was a man who had been away on a journey and had then come back home. He strutted about town, talking loudly and at great length about the brave deeds he had accomplished in the various lands he had visited. In Rhodes, the man said, he had jumped such a long jump that no man alive could equal it, and he claimed that there were witnesses who could back up his story. A bystander then remarked, 'Alright! If you're telling the truth, here is your Rhodes: go on and jump!' 

The fable shows that talking is a waste of time when you can simply provide a demonstration.

Note: This fable enjoyed popularity as a Latin proverb: 'Here's your Rhodes, jump!' (Erasmus, Adages 3.3.28).