Abu Awn al-Thaqafi said: When men of the people of the Sawad accepted Islam, Sayyiduna Umar and Sayyiduna Ali (may Allah be pleased with them both) would leave them, saying: Let them remain on their land and keep paying the kharaj.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18413