Abu Huraira reported Allah's Messenger (peace be upon him) as saying:
"...A Muslim is the brother of a Muslim.
He neither oppresses him
nor humiliates him
nor looks down upon him.
The piety is here,
(and while saying so)
he (peace be upon him) pointed towards his chest thrice.
It is a serious evil for a Muslim
that he should look down upon his brother Muslim.
All things of a Muslim are inviolable
for his brother in faith:
his blood, his wealth and his honour.
(Sahih Muslim: Book #032, Hadith #6219)