Imam Ali (PBUH):
" Do not be the slave of others for Allah ( God) had made you free."
Source: Nahjul Balagha, Letter 31
" Do not try the sharpness of your tongue against Him Who gave you the power to speak, nor the eloquence of your speaking against Him Who set you on the right path."
Source: Nahjul Balagha, Hadith n. 411
" knowledge is better than wealth. Knowledge guards you, while you have to guard the wealth. Wealth decreases by spending, while knowledge multiplies by spending, and the results of wealth die as wealth decays.