Musnad Abu Ya'la al-Mawsili 3:66

Ubayd Allah narrated to us, Sulayman ibn Dawud narrated to us, Za'idah narrated to us, from al-Suddi, from Sa'd ibn 'Ubaydah, from Abu 'Abd al-Rahman al-Sulami, who said: 'Ali delivered a sermon and said: "O people, establish the prescribed punishment (hadd) upon your slaves, those of them who are married (muhsan) and those who are not married. For a slave-girl of the Messenger of Allah, may Allah bless him and grant him peace, committed adultery, and the Messenger of Allah, may Allah bless him and grant him peace, commanded me to flog her. I went to her, and she had recently given birth. I feared that if I flogged her, she would die. So I went to the Messenger of Allah, may Allah bless him and grant him peace, and informed him, and he said: 'You have done well.'"