Ibn Abi Najih (may Allah have mercy on him) narrates from Mujahid (may Allah have mercy on him) that (the witnesses) should be two just, free Muslims, that is, the saying of Allah: 'from among those whom you approve of as witnesses' [Surah al-Baqarah: 282].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20619