What is the meaning of "disbelief in the foreignness [of language]" mentioned in the saying of the Messenger of Allah, peace and blessings be upon him: "Disbelief is in the foreignness [of language]; so if a matter becomes foreign to them, they exert themselves and innovate"?

The aforementioned hadith, "Disbelief is in the 'ajamah (non-Arabness/inarticulateness)," has no origin from the Prophet, peace and blessings be upon him. It is not found in the authenticated, narrated books. Rather, al-Tabari mentioned it with the صيغة (formulation) of "they said," without an isnad (chain of narration).

The meaning of this statement has a valid interpretation. 'Ajamah (inarticulateness) is the inability to speak clearly. What is meant here is the inarticulateness of the tongue and the lack of mastery of the Arabic language, not 'ajamah of lineage (being of non-Arab descent).

For indeed, one who has not mastered Arabic may not understand the texts of revelation well, leading him to interpret them with a false interpretation. This is one of the greatest causes of bid'ah (innovation). The people of knowledge have affirmed this: whoever is ignorant of the Arabic language will have the meanings of revelation hidden from him, and if he does not refer to the people of knowledge, he will go astray. This is the reason for the emergence of the Khawarij, the Mu'tazilah, and other innovators. The Salaf (pious predecessors) indicated that the reason for innovation is ignorance of the Arabic language.

Therefore, deductions based on ignorance of the Arabic language are considered bid'ah, because they depart from the manner of Arab speech to follow one's whims.