What is the ruling on people — like the Ash‘aris who claim to be followers of Ahlus-Sunnah Wal-Jamaa‘ah — who defame the Companions? Is it permissible to pray behind such people?
May Allaah Reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

There is no difference among scholars that it is unlawful to revile the Companions. The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “No one should revile my Companions. For if one amongst you were to spend as much gold as (the mountain of) Uhud, it would not amount to as much as one mudd (handful) of one of them or half of it.” [Al-Bukhari and Muslim]

Shaykhul-Islam said, “Al-Qaadhi Abu Ya‘la said, ‘Scholars of Fiqh are of the opinion that if the reviler of the Companions considers such an act to be lawful, he is considered a disbeliever. If he does so without considering it to be lawful, he is regarded as Faasiq (evildoer). This ruling applies whether he accuses them of disbelief or discredits their commitment to religion. A group of scholars of Fiqh in Al-Koofah and other cities are of the opinion that the reviler of the Companions should be killed.’”

Shaykhul-Islam,  may Allaah have mercy upon him, said:

Al-Qaadhi ‘Iyaadh  said, “Scholars differ regarding the defamation of the Companions. The famous opinion of the Maaliki school of Fiqh is that such a person should be punished severely. Imaam Maalik said, 'Anyone who reviles the Prophet, sallallaahu ‘alayhi wa sallam, should be killed. Anyone who reviles the Companions should be punished.' He said also, 'Anyone who reviles the Companions of the Prophet, sallallaahu ‘alayhi wa sallam, such as Abu Bakr, ‘Umar, ‘Uthmaan, Mu‘aawiyah or ‘Amr ibn Al-‘Aas should be killed if he accuses them of disbelief or deviation. However, if he reviles them with anything other than that, he should be punished severely.' Ahmad said, ‘I do not dare to kill the reviler of the Companions; however, I punish him severely.’” [As-Sawaa‘iq Al-Muhriqah]

Anyone who slanders ‘Aa’ishah, may Allaah be pleased with her, with the crime from which Allaah Has Rendered her innocent becomes a disbeliever. The one who does so belies Allaah The Almighty Who Said that she is chaste and there is no difference among scholars regarding this ruling.

Prayer behind those who defame the Companions is valid as long as such defamation does not result in the disbelief of the person. Most scholars see that praying behind a man who commits an innovation in religion is valid but it is disliked. So, it is more appropriate to abandon prayer behind him, as an act of forbidding evil, if there is another Imaam who follows Ahlus-Sunnah Wal-Jamaa‘ah to pray behind. Scholars of Fiqh are in agreement that those who lead the prayer should be among the best people.

We bring to your attention the fact that the Ash‘aris do not defame the Companions; rather, they dignify them like Ahlus-Sunnah Wal-Jamaa‘ah. Their sayings in praising and favoring the Companions are well-known and published in their books. 

Allaah Knows best.