Ne in Nervum erumpat.

The string may break; this was said to persons who, emboldened by success, were perpetually engaging in new exploits: such persons were advised by this apothegm to desist, they had done enough to shew their skill or courage; a reverse might happen, or by one wrong step they might lose all the honour or emolument they had gained. The pitcher that goes often to the well returns broken at last.
The adage takes its rise from bowmen who, by overstraining the string, at length occasion it to break, not without danger to themselves.