عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ عَنْ إِخْصَاءِ الْخَيْلِ وَالْبَهَائِمِ وَقَالَ ابْنُ عُمَرَ فِيهَا نِمَاءُ الْخَلْقِ
It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) forbade the castration of horses and other animals. Sayyiduna Ibn Umar (may Allah be pleased with them both) said: Not castrating increases the progeny.