Musannaf Ibn Abi Shayba 13:1578

Abu Bakr narrated to us, he said: Humayd narrated to us, from Hasan, from Mansur, from Ibrahim, who said: If a man from the people of As-Sawad embraces Islam, then remains in his land, the land tax (kharaj) is taken from him. If he leaves it, the land tax is not taken from him.