Dear Zeinab Matlubi you sould to methylated of ABA in solution of dry diazimetan with fresh medical ether without peroxides. After you sould evaporate of ether and before you can use to analysis after disolve in ethanol. In a dry form, after ether, ABA can storade about 10-15 day.
Do you mean a working solution of the commercial reagent? For biological experiments usually in ethanol solution at -20 °C, but not for long. See here under Handling: http://www.abcam.com/-Abscisic-acid-ab120860.html