Question:
The questioner says, may Allah reward you well: What is the ruling on an Ismaili praying with Ahl al-Sunnah (the People of the Sunnah)?
Answer:
It depends on their beliefs; a person is looked at—whether they are Ismaili or Rafidi (Shiite)—and if their belief contradicts the belief of Muslims, it is not valid. It is well known that the Ismailis are Batinis (esoteric sects) and that their beliefs are matters of disbelief (kufr), and likewise the Rafida. Therefore, praying behind them is not valid.
As for praying with them (meaning alongside them in congregation where their specific status is unknown): the hypocrites used to pray with the Prophet ﷺ and with the Muslims. If a person’s condition is not known and they conceal their disbelief, rulings are applied based on outward appearances.
As for one who openly displays his disbelief and hypocrisy, he is referred to the Sharia court: he is asked to repent, and if he repents, [he is left alone], otherwise he is executed. But as for the one who conceals his disbelief, he remains with the Muslims, and the outward rulings of Islam are applied to him.