صحيح البخاري

Narrated Abu Huraira:

I said, "O Allah's Messenger! I am a young man and I am afraid that I may commit illegal sexual intercourse and I cannot afford to marry." He kept silent, and then repeated my question once again, but he kept silent. I said the same (for the third time) and he remained silent. Then repeated my question (for the fourth time), and only then the Prophet said, "O Abu Huraira! The pen has dried after writing what you are going to confront. So (it does not matter whether you) get yourself castrated or not."