A bad conduct only spoils a good deed just as vinegar spoils honey (Usul Al-Kafi, Vol. 2, p. 32).
Do not be misled into thinking that a great deal of knowledge without correcting and cultivating one’s conduct can be of any use.
Cultivating manners/ethics is the key to everything good, the source of everything beautiful, the one which brings about every fruit, the basis of any objective. Look how even the unbelievers are rewarded on account of their good manners…
The above are some of the interesting quotes taken from chapter 1 of ‘the lantern of the path to Allah almighty’