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Following the Islamic Conquest of Persia (Iran), there were important changes in the role of the different dialects within the Persian Empire. The old prestige form of Middle Iranian, also known as Pahlavi, was replaced by a new standard dialect called Dari as the official language of the court. The name Dari comes from the word darbâr (دربار), which refers to the royal court, where many of the poets, protagonists, and patrons of the literature flourished. The Saffarid dynasty in particular was the first in a line of many dynasties to officially adopt the new language in 875 CE. Dari may have been heavily influenced by regional dialects of eastern Iran, whereas the earlier Pahlavi standard was based more on western dialects. This new prestige dialect became the basis of Standard New Persian. Medieval Iranian scholars such as Abdullah Ibn al-Muqaffa (8th century) and Ibn al-Nadim (10th century) associated the term "Dari" with the eastern province of Khorasan, while they used the term "Pahlavi" to describe the dialects of the northwestern areas between Isfahan and Azerbaijan, and "Pârsi" ("Persian" proper) to describe the Dialects of Fars. They also noted that the unofficial language of the royalty itself was yet another dialect, "Khuzi", associated with the western province of Khuzestan.

What religion slowed the spread of the Iranian language?
unanswerable