Sahih Muslim Book 17, Hadith Number 4224.

Chapter : The prescribed punishment should be deferred in case of one who is in a state of parturition.

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.