He also said—may Allah have mercy on him:

If a flock numbers forty animals, whether young or adult, zakat is due on them once a year has elapsed. If they initially numbered fewer than forty but number forty at the end of the year, there is disagreement on this; the more cautious course is to pay zakat. And Allah knows best.

He was asked—may Allah have mercy on him:

A man owns a flock that has not reached the minimum threshold for zakat. Does zakat become due on it during the course of the year?

He answered:

The scholars hold two views on this issue, both reported from Ahmad:

The first is that the year begins when the flock reaches forty, as al-Shāfiʿī held.

The second is that the year begins when he acquired the dams, as Mālik held. And Allah knows best.