Dear please follw the following procedure 

Solid DAB must be dissolved first in a small volume of water, then diluted with a buffer

(usually phosphate or TRIS, pH 7.2/7.6). for further information kindly read this

https://www.researchgate.net/publication/9059139_Stability_and_solubility_of_33'-diaminobenzidine_DAB

Article Stability and solubility of 3,3'-diaminobenzidine (DAB)

Thank you Athar Farooq