Muwatta Imam Malik Book of Zakat, Hadith 27

Yahya said that Malik said, "The position with us concerning a man who has zakat to pay on one hundred camels but then the zakat collector does not come to him until zakat is due for a second timeand by that time all his camels have died except five, is that the zakat collector assesses from the five camels the two amounts of zakat that are due from the owner of the animals, which in this case is only two sheep, one for each year. This is because the only zakat which an owner of livestock has to pay is what is due from him on the day that the zakat is (actually) assessed. His livestock may have died or it may have increased, and the zakat collector only assesses the zakat on what he (actually) finds on the day he makes the assessment. If more than one payment of zakat is due from the owner of the livestock, he still only has to pay zakat according to what the zakat collector (actually) finds in his possession, and if his livestock has died, or several payments of zakat are due from him and nothing is taken until all his livestock has died, or has been reduced to an amount below that on which he has to pay zakat, then he does not have to pay any zakat, and there is no liability (on him) for what has died or for the years that have passed.