Sahih Bukhari
Wedlock, Marriage (Nikaah)
Volume: 007 / Book Number: 062 / Hadith Number: 9
Narrated Ibn Masud:
We used to fight in the holy battles in the company of the Prophet and we had no wives with us. So we said, "O Allah's Apostle! Shall we get castrated?" The Prophet forbade us to do so.