Ubayd bin Ziyad al-Hadrami (may Allah have mercy on him) said: Malik bin Dinar (may Allah have mercy on him) met Salim bin Abdullah bin Umar bin al-Khattab (may Allah have mercy on him), while he was riding a donkey whose ears drooped, and his saddle and clothes were worn out. Salim asked him: Whose man are you? He said: I am from you, and I belong to you, and I am one of your mawali (freed clients). Then he said: Narrate to me a hadith of the Messenger of Allah (peace and blessings be upon him). (So Salim related that the Messenger of Allah (peace and blessings be upon him) said:) "A Muslim is the brother of a Muslim; he does not wrong him, nor betray him, nor abandon him helpless in a calamity that befalls him. If the best of the Arabs and the mawali come together, they love one another with a love they cannot avoid; and if the worst of the two groups come together, they hate one another with a hatred they also cannot avoid."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13182