The second and third caliphs, Umar ibn al-Khattab and Usman ibn Affan, continued Abu Bakr's codification of the zakat. Uthman also modified the zakat collection protocol by decreeing that only "apparent" wealth was taxable, which had the effect of limiting zakat to mostly being paid on agricultural land and produce. During the reign of Ali ibn Abu Talib, the issue of zakat was tied to legitimacy of his government. After Ali, his supporters refused to pay zakat to Muawiyah I, as they did not recognize his legitimacy.