Musnad Ahmad 26:159

Yazid narrated to us, Ismail narrated to us, from Qays, from Ibn Mas’ud, who said: We used to go on military expeditions with the Prophet, peace and blessings of Allah be upon him, while we were young men, and we did not have wives. So we said: "O Messenger of Allah, shall we castrate ourselves?" He forbade us from that.