Abd al-Rahman reported that 'Ali, while delivering the address said:
O people, impose the prescribed punishment upon your slaves, those who are married and those not married, for a slave-woman belonging to Allah's Messenger (ﷺ) had committed adultery, and he committed me to flog her. But she had recently given birth to a child and I was afraid that if I flogged her I might kill her. So I mentioned that to Allah's Apostle (ﷺ) and he said: You have done well. (Sahih Muslim 1705)
I didn't find any Hadith to establish that we need four witnesses to convict a slave for committing fornication or adultery. The Hadith above is silent on this. Is this requirement sought? Did the Prophet SAW punished the slave after hearing the testimony of four trustworthy witnesses? Or is it reserved only for the free?