Allah, the Blessed and Exalted, said concerning slave-women: "And when they are married, then if they commit an indecency, upon them is half of the punishment that is upon free (married) women." [Surah al-Nisa: 25]
Imam al-Shafi'i (may Allah have mercy on him) said: "Half can only be in the punishment of flogging, which can be divided; as for stoning, which is killing, it has no half."
He (further) said: "And the Messenger of Allah (peace and blessings be upon him) said: 'When the slave-woman of one of you commits zina and her zina becomes clear, let him flog her.' And he (peace and blessings be upon him) did not say that he should stone her."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / Q17086