Musnad Ahmad 26:752

Yahya ibn Zakariyya narrated to us, he said: Isma'il informed me, from Qays, from Ibn Mas'ud, who said: We were with the Messenger of Allah, may Allah bless him and grant him peace, and we had no women. We said: "O Messenger of Allah, shall we castrate ourselves?" He forbade us from that, and said: "O you who have believed, do not prohibit the good things which Allah has made lawful to you," [Al-Ma'idah: 87] to the end of the verse.