Musnad Ahmad 32:957

Waki’ narrated to us, al-A’mash narrated to us, from Sahl Abu al-Asad, from Bukayr al-Jazari, from Anas, who said: We were in the house of a man from the Ansar, and the Messenger of Allah (peace and blessings of Allah be upon him) came until he stood and took hold of the two doorposts. He said: "The Imams are from Quraysh. They have a right over you, and you have the like of that, provided that when they are asked for mercy, they show mercy; when they judge, they act justly; and when they make a covenant, they fulfill it. Whoever among them does not do that, then upon him is the curse of Allah, the angels, and all of mankind."