Ali traced this back to the Prophet ﷺ, who said, "The leaders are from Quraysh. When mercy is sought from them, they show mercy. When they make a covenant, they fulfill it. When they judge, they judge justly. Whoever among them does not do this, upon him is the curse of Allah, the angels, and all people."
يرفعه إلى النبي ﷺ قال: الأئمة من قريش إذا استرحموا رحموا، وإذا عاهدوا وفوا، وإذا حكموا عدلوا، فمن لم يفعل ذلك منهم فعليه لعنة الله والملائكة والناس أجمعين.
Ahmad (19777) narrated it from Sulayman bin Dawud, that is, Tayalisi, he said, Sukayn told us, Sayyar bin Salama told us, he heard Abu Barza attributing it to the Prophet ﷺ, and he said, and then he mentioned it. Ahmad (19805) and Abu Yaala (3645) also narrated it, both through two other routes, from Sukayn bin Abdul-Aziz, from Sayyar bin Salama Abu Minhal Riyahi, who said, I entered along with my father upon Abu Barza Aslami, and at that time I had two earrings in my ears, and I was a young boy. He said, Abu Barza said, "I praise Allah that I have woken up blaming this clan of Quraysh, so-and-so here fighting over this world, and so-and-so here fighting over this world," meaning Abdul-Malik bin Marwan. He said, until he mentioned Ibn Azraq. He said, then he said, "The most beloved people to me are this band whose bellies are lean from the wealth of the Muslims and whose backs are light from their blood." He said, the Messenger of Allah ﷺ said, "The commanders are from Quraysh, the commanders are from Quraysh, the commanders are from Quraysh. I have a right over them and they have a right over you, so long as they do three things, so long as they judge and act justly, and when asked for mercy show mercy, and when they make a covenant they fulfill it. Whoever among them does not do that, upon him be the curse of Allah, the angels, and all the people."Its chain of transmission is hasan, because of Sukayn bin Abdul-Aziz Abdi Attar Basri, concerning whom there is disagreement. Waki, Ibn Main, and Nasai deemed him reliable. Ibn Khuzayma said, "I do not know him, and I do not know his father," and he said elsewhere, "I disclaim responsibility for him and for his father." I say, his student Ibn Hibban knew him, as did others before them who knew him, and they either deemed him reliable or deemed him daif. The weakness may lie with whoever narrated from him, as Ibn Adi said, otherwise he is not problematic.