Amr bin Shu'ayb narrates from his father, from his grandfather, that the Prophet (peace and blessings be upon him) said: "The testimony of a treacherous man, a treacherous woman, one who harbours rancour against his brother, and a slave in favour of his master shall not be accepted; regarding others it is permissible."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20854