Muwatta Malik, Book 51, Hadith 4 (USC-MSA) ⓘ
Yahya related to me from Malik from Nafi that Abdullah ibn Umar disapproved of castration and said, "The completeness of the created form is in the testicles."
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يَكْرَهُ الإِخْصَاءَ وَيَقُولُ فِيهِ تَمَامُ الْخَلْقِ .
also referenced under: Book 51, Hadith 1736 (In-Book)