מדרש אויף יחזקאל 33:2

Variantly: "and I shall set them at your heads": If you guard your ways, your heads (i.e., your judges) will be guarded, and if not, your heads will not be guarded — whence we are taught that the guilt of Israel is on the heads of their judges. And thus is it written (Ezekiel 33:7-9) "A sentinel have I made you for the house of Israel. When you hear a word from My mouth, you must warn them for Me." (If you do not warn the wicked one and he dies, his blood will be on your head.)