مسند امام احمد بن حنبل · Hadith 4302

It was narrated that lbn Mas'ood said: We were with the Messenger of Allah (PBUH) and we had no wives with us. We said: O Messenger of Allah, can we not castrate ourselves? But he forbade us to do that and said: "O you who believe! Make not unlawful the Tayyibat (all that is good as regards foods, things, deeds, beliefs, persons) which Allah has made lawful to you, and transgress not. Verily, Allah does not like the transgressors" [al-Ma'idah 5:87].