It is reported from Sayyiduna Umar (may Allah be pleased with him) that he intended to divide the people of al-Sawad among the Muslims and ordered that they be counted. They found that each Muslim man would receive three peasants (non-Arab farmers). He consulted the Companions of the Prophet (peace and blessings be upon him) about it, and Sayyiduna Ali (may Allah be pleased with him) said: Leave them, so that they remain a source of supply for the Muslims. So he sent Uthman bin Hunayf (may Allah be pleased with him), who imposed on them (a tax of) 48, 24 and 12 (dirhams).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18370