Musnad Ahmad 26:563

Waki’ narrated to us, from Ibn Abi Khalid, from Qays, from Abdullah, who said: We were with the Prophet, peace and blessings of Allah be upon him, and we were young men, so we said: "O Messenger of Allah, shall we not castrate ourselves?" He forbade us from that, then he granted us a concession to marry a woman for a garment for a specified term. Then Abdullah recited: "Do not forbid the good things which Allah has made lawful for you." [Al-Ma'idah: 87]