Musnad Ahmad 26:103

Yahya narrated to us, Ismail—who is Ibn Abi Khalid—narrated to us, Qays narrated to me, from Ibn Mas’ud, who said: We used to go on military expeditions with the Messenger of Allah, may Allah bless him and grant him peace, and we did not have our wives with us. So we said: "O Messenger of Allah, shall we castrate ourselves?" He forbade us from that.