The Peach Blossom Spring was written during a time of political instability and national disunity. The story describes how a fisherman haphazardly sailed into a river in a forest made up entirely of blossoming peach trees, where even the ground was covered by peach petals. When he reached the end of the river (or spring in some translations), the source turned out to be a grotto. Though narrow at first, he was able to squeeze through and the passage eventually reached a village with animals and people of all ages.
The villagers were surprised to see him, but were kind and friendly. They explained that their ancestors escaped to this place during the civil unrest of the Qin dynasty and they themselves had not left since or had contact with anyone from the outside. As a result, they had heard nothing of subsequent changes in political regimes.
The fisherman was warmly received by the hospitable villagers and stayed for over a week. Upon leaving, he was informed that it was worthless to reveal this experience to the world. However, he marked his route on his way out with signs and later divulged the existence of this idyllic haven to others. They tried to find it repeatedly but in vain.

Question: What was the fisherman warned when he left the village?
Answer: That it would be worthless to reveal his experience to the world.
Alternative answer: Not to tell of its existance.