Musnad Ahmad 28:422

'Abd al-Razzaq narrated to us, Muhammad ibn Rashid narrated to us, from Sulayman ibn Musa, from 'Amr ibn Shu'ayb, from his father, from 'Abdullah ibn 'Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The testimony of a treacherous man or a treacherous woman is not permissible, nor [the testimony] of one who holds a grudge against his brother. And the testimony of the *qani'* (dependent) for the people of the house is not permissible, but his testimony for others is permissible." And the *qani'* is the one whom the people of the house provide for.