Muhammad bin Qays al-Asadi narrates from Imam al-Sha'bi (may Allah have mercy on him) that he was asked, in the time of Umar bin Abd al-Aziz (may Allah have mercy on him), about the people of al-Sawad: Did they have a covenant? He said: They had none; but when kharaj (land tax) began to be taken from them with consent, they came to have a covenant.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18367