مسند أحمد

It was narrated that Harithah said: Some people from Syria came to ‘Umar and said: We have acquired wealth and horses and slaves,and we want to pay zakah on them as a purification. He said: I shall do what my two predecessors did. He consulted the Companions of Muhammad (ﷺ), among whom was ‘Ali, and ‘Ali said: It is good, provided it does not become a regular tax that is taken from them after you are gone.