Nafi' narrates from Ibn Umar (may Allah be pleased with him) that he disliked castrating livestock and used to say: "Do not make changes in the creation of Allah."
This is the correct version, mawquf (stopping at the Companion), though it has also been narrated as marfu' (attributed to the Prophet).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السبق والرمي / 19794