Musnad Ahmad 5:118

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, I heard ‘Abd al-A‘la narrating from Abu Jamilah, from ‘Ali: That a slave-girl of theirs committed adultery and became pregnant. ‘Ali came to the Prophet, may Allah bless him and grant him peace, and informed him. He said: "Leave her until she gives birth or delivers, then flog her."