Sahih Muslim
The Book Pertaining to Punishments Prescribed by Islam (Kitab Al-Hudud)
Book Number: 017 / Hadith Number: 4224
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 (may peace be upon him) 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 (may peace be upon him) and he said: You have done well.