Ibn Shihab (may Allah have mercy on him) said: The practice of Islam has been established that the testimony of a quarrelsome person and of an untrustworthy person shall not be accepted, and the testimony of a disputant shall not be accepted regarding his opponent.
The rejection of the testimony of one under suspicion has been narrated to us in general terms through two mursal routes from the Prophet (peace and blessings be upon him), and through another route as connected (mawsul), although there is weakness in it; it is strengthened by the two mursal routes along with it. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20866