Muwatta Imam Malik (Riwayat Yahya)

Imam Malik, may Allah have mercy on him, said: If a person has less than the nisab of goats, then he buys more goats or inherits some, so that they become more than the nisab, then zakat will not be obligatory upon him until a year has passed from the date of purchase or inheritance. The reason for this is that when a person has such a number of animals—camels, cows, or goats—in which there is no zakat, then this will not be counted as nisab until the number of each type of animal reaches the nisab. If the amount reaches the nisab, then whatever animals of that type are added to it, zakat will have to be given on them along with that nisab, whether these animals are few or many.