What is the meaning of the the Prophet's (PBUH) saying to Abu Hurairah: "So either stick to that or leave it"?

The authentic prophetic hadiths have forbidden castration, and it is unanimously prohibited for human beings. Al-Bukhari and Muslim narrated from Ibn Mas'ud, who said: "We used to go on campaigns with the Prophet, may Allah bless him and grant him peace, and we had no women with us. So we said: 'O Messenger of Allah, should we not castrate ourselves?' He forbade us from doing that."

As for the hadith of Abu Hurairah: «O Abu Hurairah, the pen has dried with what you will encounter, so castrate yourself on that basis or leave it,» it is not a permission for castration. Rather, it came as a form of threat and warning, and its essence is that all matters are by Allah's decree, so castration and leaving it are the same, for what has been decreed must inevitably occur.