Muwatta Malik #1860
Hair
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
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."
About Muwatta Malik
This hadith is #1860 from Muwatta Malik, in the Book of Hair.