If by stability you talk about degradation, then the more concentrated the fastest the degradation.

Thanks Mr. Gonzalez-ortega for your answer. can you explain more this solution behavior; why concentrated solutions degrade faster than diluted ones? and is that general or it depends on the type of compound dissolved in a defined solvent??

Dilute samples are less stable.

It depends on what you mean by "stability": that due to the inherent chemistry of the solute, that due to interactions with a solvent, or that due to external factors like temperature or sunlight. They find jars with vinegar at the bottom of the sea in ancient Greek ships, and the vinegar is still vinegar. You can call it a dilute acetic acid aqueous solution. On the other hand, hydrogen peroxide solutions tend to be more stable when they are more concentrated (you'd still better keep them in a fridge and away from sunlight). Then sodium chloride solution for instance will stay sodium chloride at any concentration just about forever. Then of course things may be completely different with other solvents: one reagent may completely degrade in water (hydrolyze), but be perfectly stable in polar aprotic solvents, etc. It very much depends on the overall chemistry of the system. There is no simple "yes" or "no" answer here.