Al-Sha'bi (may Allah have mercy on him) said: The ihsan of a slave-woman is her entering Islam and affirming it; when she has entered Islam and affirmed it and then commits zina, she is to receive fifty lashes. Ibrahim (may Allah have mercy on him) interprets "when they are muhsan" [Surah al-Nisa: 25] as becoming Muslim, and Mujahid (may Allah have mercy on him) as marriage, and he says: If she has not been married, there is no hadd upon her. Mujahid (may Allah have mercy on him) also narrates the like of this from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17095