Sahih Ibn Khuzaymah

Hazrat Harithah bin Mudarrib narrates that some people from Syria came to Sayyiduna Umar (may Allah be pleased with him) and said, "Indeed, we have horses and slaves, and we wish that zakat be collected from these assets of ours." Upon this, Sayyiduna Umar (may Allah be pleased with him) said, "This was not done by my two companions before me, so how can I do it?" Then he consulted the noble companions of the Prophet. Among them was Sayyiduna Ali (may Allah be pleased with him), who said, "This is a very good thing, provided that it does not become a continuously collected jizyah." Imam Abu Bakr (may Allah have mercy on him) says that the Sunnah of the Prophet is that there is no zakat on four camels unless the owner himself wishes to pay it voluntarily, in which case it will be accepted. Regarding goats, his noble statement is: "When a person's grazing goats are even one less than forty, then there is no zakat on them unless the owner wishes to pay it voluntarily." And for silver, the zakat is one-fortieth. Then, if the silver is only one hundred and ninety dirhams, there is no zakat on it unless the owner wishes to pay it voluntarily. This is evidence that when the owner of wealth pays zakat on his wealth voluntarily, even though zakat is not obligatory on it, the Imam can accept their zakat. Similarly, when Sayyiduna Umar Farooq (may Allah be pleased with him) informed the people of Syria that neither the Prophet Muhammad (peace be upon him) nor Sayyiduna Abu Bakr Siddiq (may Allah be pleased with him) collected zakat on horses and slaves before him, and then they wished to pay their voluntary zakat, it became permissible for Sayyiduna Umar Farooq (may Allah be pleased with him) to collect zakat from their wealth. Just as the Prophet Mustafa (peace be upon him) declared it permissible to collect zakat on less than five camels, less than forty goats, and less than two hundred dirhams of silver.