Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that he heard the Messenger of Allah (peace and blessings be upon him) say: "The testimony of a Bedouin against a town-dweller is not permissible." This concerns the reliability of testimony: the witness should also have knowledge of inner matters. Shaykh Abu Sulayman al-Khattabi (may Allah have mercy on him) said that the testimony of a Bedouin is disliked because of the harshness in religion and the ignorance of the rulings of the Shari'ah found among them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21182