مُوَطَّأُ مَالِكٍ
Muwatta Malik 1736
Hair
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ كَانَ يَكْرَهُ الإِخْصَاءَ وَيَقُولُ فِيهِ تَمَامُ الْخَلْقِ .
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."
Arabic/English book reference: Book 51, Hadith 4