Bukhari

Hadith Commentary:
A kid goat will be taken in zakat when the collector deems it appropriate, or if a person is left with nothing but kids.

Imam Bukhari, in the heading of the hadith, derived this indication from the words of Abu Bakr as-Siddiq radi Allahu anhu: "If these people refuse to give even a kid goat which they used to give during the time of the Messenger of Allah sallallahu alayhi wa sallam, I will fight them for it."

In the beginning, Umar radi Allahu anhu hesitated to fight those who withheld zakat, because they professed the shahada.

But Abu Bakr radi Allahu anhu had more knowledge than him regarding this matter.

Eventually, Umar radi Allahu anhu also agreed with him.

It is clear from this hadith that merely reciting the shahada does not complete a person's Islam.

Until one accepts all the principles and definitive obligations of Islam.

If someone denies any definitive obligation of Islam—such as prayer (salah), fasting (sawm), zakat, jihad, or hajj—he becomes a disbeliever, and it is permissible to fight against him.

(Wahidi)

Hadith Commentary:
(1)
A kid goat that is four months old and does not drink milk but rather eats fodder—if it is male, it is called "jadi," and if it is female, it is called "anaaq."

(2)
Imam Bukhari rahimahullah has indicated that if the collector of zakat wishes, he may take a kid goat, because there is no defect in it. It is far better than a defective animal. This is why Imam Bukhari has used the word "taking" (akhdh) instead of "giving" (i‘taa’) in this chapter heading, meaning that doing so is left to the discretion of the one collecting the charity. (Fath al-Bari: 3/405)

(3)
The jurists have described the scenario as follows: if a person has only forty kid goats of four months of age, and zakat becomes obligatory on them, then one kid goat will be taken from them. The situation in which zakat becomes obligatory is as follows: a person had two hundred goats which gave birth to kids, but due to some calamity, the goats died and only forty four-month-old kids remained. Since zakat had already become obligatory on them from the beginning of the year, and in such a case, the passage of a year is not necessary for the kids, because they take the ruling of their mothers, therefore, one kid goat will be taken from them. And Allah knows best.