It is reported from Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) regarding the statement of Allah Almighty: "The free for the free, the slave for the slave, and the female for the female" [Surah al-Baqarah: 178], that (each) is to be killed (in retaliation). He said: A man used not to be killed for a woman; rather, a man was killed for a man and a woman for a woman. Then Allah Almighty revealed this verse: "A life for a life" [Surah al-Ma'idah: 45]. So the free, men and women, became equal in retaliation, and slaves, men and women, became equal, in that it is a life for a life.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15969