Musnad Abu Ya'la al-Mawsili 160:49

Ahmad ibn ‘Isa narrated to us, ‘Abdullah ibn Wahb narrated to us, he said: Yahya ibn Ayyub informed me, from Ibn al-Had, from Muhammad ibn ‘Amr, from ‘Ata’ ibn Yasar, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The testimony of a Bedouin against a town-dweller is not accepted."