حَدَّثَنَا وَكِيعٌ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نَافِعٍ عَنْ أَبِيهِ عَنِ ابْنِ عُمَرَ قَالَ نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنْ إِخْصَاءِ الْخَيْلِ وَالْبَهَائِمِ و قَالَ ابْنُ عُمَرَ فِيهَا نَمَاءُ الْخَلْقِ
مسند امام احمد بن حنبل · Hadith 4769
It was narrated that Ibn 'Umar said; The Messenger of Allah (PBUH) forbade castrating horses and other animals. And Ibn 'Umar said: Leaving them intact is the means of reproducing and increasing them in number.
Translated by Dr. Muhammad Muhsin Khan · Reviewed by Islamic University of Madinah · Published March 2020