The Messenger of Allah ﷺ forbade the castration of horses and other animals. Ibn Umar said, "In them lies the increase of creation."
حدثنا وكيع حدثنا عبد الله بن نافع عن أبيه
نهى رسول الله ﷺ عن إخصاء الخيل والبهائم وقال ابن عمر فيها نماء الخلق
No references are available.
Its isnad is daif. Abdullah bin Nafi, the freed slave of Ibn Umar, is extremely daif. Bukhari said in al-Duafa, page 21, "His hadith is munkar." Abu Hatim said the same. Bukhari said in al-Tarikh al-Saghir, page 163, "He contradicts others in his hadith," and on page 179, "There is something questionable about him." Nasai said in al-Duafa, page 19, "His hadith is to be abandoned." Ibn Hibban said, "He used to make mistakes without realizing it, so his reports that do not agree with what the trustworthy narrators report cannot be relied upon."The hadith appears in Majma al-Zawaid 5:265, where it is stated, "Ahmad narrated it, and it contains Abdullah bin Nafi, who is daif."Ikhsaa (castration) - this is how it appears in both original copies, as a four-letter root form. What appears in the dictionaries is "khisaa," with a kasrah on the kha and a long vowel, as a three-letter root form. This is also the form found in Majma al-Zawaid.