Yunus bin Abd al-A'la (may Allah have mercy on him) said: I heard Imam al-Shafi'i (may Allah have mercy on him) saying: The testimony of the people of desires (ahl al-ahwa') is permissible, but the testimony of the Rafidah is not permissible, because they look after one another.
The Shaykh (may Allah have mercy on him) said: Likewise, whoever among them is known for reviling the Companions, who are the lamps of this ummah and its foremost, his testimony will not be accepted, whenever his reviling of them stems from partisanship or ignorance, and not from an interpretation or a doubt.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20906