From Sahih Bukhari (Hadith 5071):
Narrated Ibn Masud:We used to fight in the holy battles in the company of the Prophet (ﷺ) and we had no wives with us. So we said, "O Allah's Messenger (ﷺ)! Shall we get castrated?" The Prophet (ﷺ) forbade us to do so
Reference: Bukhari 5071