What is the ruling on one who slanders or speaks ill of any of the Companions, among those who claim to be from Ahl as-Sunnah, such as the Ash'aris, and is it permissible to pray behind him?

It is forbidden to insult the Companions by scholarly consensus. Whoever insults them considering it permissible has committed an act of disbelief (kufr). If one insults them without considering it permissible, they are considered a transgressor (fasiq) but not a disbeliever. Some jurists have gone so far as to rule that whoever insults the Companions should be executed. Whoever slanders Aisha with what Allah has cleared her of has committed an act of disbelief. It is permissible to pray behind someone who insults the Companions if their insult does not amount to disbelief, though it is disliked. It is preferable not to pray behind them if someone else is available. The Ash'aris venerate the Companions and do not insult them.