Musnad Ahmad 789:15

Sulayman ibn Dawud narrated to us, Sakin narrated to us, Sayyar ibn Salamah narrated to us, he heard Abu Barzah attributing it to the Prophet, may Allah bless him and grant him peace, who said: "The leaders are from Quraysh: if they are asked for mercy, they show mercy; if they make a covenant, they fulfill it; and if they judge, they act justly. Whoever among them does not do that, then upon him is the curse of Allah, the angels, and all of mankind."