ʿAbdullah b. Zayd Al-Numayrī reports that Al-Ḥasan Al-Baṣrī said:

They (the heretics) were destroyed by their inability in Arabic (al-‘ujmah).

Al-Bukhārī, Al-Tārīkh Al-Kabīr Vol.5 p99.

And Imām Al-Shāfi’ī said:

People didn’t become ignorant and didn’t differ amongst themselves except because they left Arabic and leaned towards the language of Aristotle.

Quoted by Al-Suyūṭī in Ṣawn Al-Manṭiq p15. He said on p22:

I have found Salaf before Al- Shāfi’ī indicate what he did: that the cause of heresy (al-ibtidā’) is ignorance of Arabic language.