Imaam Nawawi (rahimahullaah) said:

”As for rebelling against them (i.e. the rulers) and fighting them, it is forbidden by way of consensus of the Muslims (i.e. scholars of Islam) even if they (i.e. rulers) are sinful and oppressive.  And indeed the ahaadeeth are manifestly clear regarding what we have mentioned and Ahlus Sunnah (i.e. the Ulama of Ahlus Sunnah) have consensus that the ruler is not removed due to (him being) sinful.” [Sharh An-Nawawi 12/229]