Sayyidah Aishah (may Allah be pleased with her) said: Her husband was a slave, so the Messenger of Allah (peace and blessings be upon him) gave her the choice, and she chose herself (separation). Had her husband been free, he (peace and blessings be upon him) would not have given her the choice.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14259