Musnad Ahmad 5:771

Sulayman ibn Dawud narrated to us, Zaidah informed us, from al-Suddi, from Sa'd ibn 'Ubaydah, from Abu 'Abd al-Rahman al-Sulami, who said: 'Ali, may Allah be pleased with him, addressed us and said: "O people, establish the prescribed punishments (hudud) upon those whom you possess (your slaves), both those among 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 carry out the punishment upon her. I went to her and found that 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 mentioned that to him. He said: 'You have done well.'"