Ibn Abi Najih (may Allah have mercy on him) narrates from Mujahid (may Allah have mercy on him) that in the saying of Allah, the Exalted, 'And take two witnesses from among your men' [Surah al-Baqarah: 282], 'men' (rijal) means free people.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20607