Islam prohibits harming animals. Mercy, in Islam, should dominate all aspects of life.
The late Sheikh Sayyed Mutawalli Ad-Darsh, former chairman of the UK Shari`ah Council (may Allah have mercy on him) states: “Neutering or sterilizing animals, while not encouraged in Islam, is not completely forbidden either. `Abdullah ibn `Umar, a Companion of the Prophet (peace and blessings be upon him) reported that the Prophet forbade the neutering of horses and other animals.
However, according to another tradition, the Prophet (peace and blessings be upon him) is said to have permitted the sterilization of an animal so long as the operation is carried out early in its life and not when the animal reaches maturity. Hence, it may be acceptable to neuter pets such as cats, particularly if one wants to prevent the birth of a multiplicity of unwanted kittens.”
Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, adds:
“Preventing your pet from reproducing is preventing a natural process that Allah has created in it. Undoubtedly the rulings on animals are not as serious as in the case of humans, but this does not mean violating the rights of the creation of Allah. If this operation will cause harm, or it will cause complications for the pet, then it is not permissible. The prohibition against causing harm is general and includes harm against both humans and animals. Below are some of the hadiths that indicate this:
It was narrated from Ibn ‘Umar (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “A woman entered Hell because of a cat which she tied up and did not feed, nor did she allow it to eat of the vermin of the earth.” The vermin of the earth means mice, etc. (Reported by Al-Bukhari and Muslim). It was narrated from Jabir ibn ‘Abd Allah that a donkey whose face had been branded passed by the Prophet (peace and blessings of Allah be upon him), and he said, “May Allah curse the one who branded it.” (Reported by Muslim). The scholars differed concerning the neutering of non-humans as follows:
The Hanafi scholars said that there is nothing wrong with neutering animals, because that benefits both the animals and humans.
The Maliki scholars said that it is permissible to neuter animals whose meat may be eaten, and it is not makruh (reprehensible), because that makes the meat better.
The Shafi`i scholars made a distinction between animals whose meat is edible and others. They said that it is permissible to neuter animals when they are small if they are animals whose meat is edible, but it is haram (unlawful) in other cases. They also stipulated the condition that this neutering should not cause the animal’s death.
According to the Hanbali scholars, it is permissible to neuter sheep because that makes the meat better, but it was said that it is makruh in the case of horses, etc.
Imam Ahmad said: I do not like for a man to neuter anything, rather that is makruh because of the prohibition on causing pain to animals.
Therefore, we say that if there is some benefit in neutering pets and if that will not cause its death, then it is permissible.”