Abdullah bin Abi Bakr bin Muhammad bin Amr bin Hazm narrated from his father, who narrated from his grandfather, that the Prophet Muhammad (peace be upon him) wrote a letter for them ......... and narrated the hadith similar to the above.
From these hadiths, the minimum threshold (nisab) for the zakat of goats is known, which is forty; there is no zakat on less than this. And if there are more than four hundred goats, then for every additional hundred, one goat is to be added. And Allah knows best.