Imam al-Shafi'i (may Allah have mercy on him) says regarding this verse, and Allah knows best what He meant by it: I have heard someone who interprets this verse as meaning (two others) from other than your tribe among the Muslims. He takes as evidence the saying of Allah: 'You shall detain them both after the prayer, and they shall swear by Allah, if you are in doubt: We will not sell it for a price' [Surah al-Ma'idah: 106], and the prayer at fixed times is for the Muslims; and the saying of Allah: 'even if he be a near relative' [Surah al-Ma'idah: 106], and kinship existed only among the Arab Muslims who were with the Prophet (peace and blessings be upon him), or between them and the idol-worshippers, not between them and the dhimmis. And Allah says: 'And we will not conceal the testimony of Allah, for then we would indeed be among the sinners.' [Surah al-Ma'idah: 106] It is only Muslims who feel sinful about concealing testimony for Muslims, not the dhimmis.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20620