Benefits and Issues: The Messenger of Allah (sallallahu alayhi wa sallam) was the most eloquent of the Arabs. And his (sallallahu alayhi wa sallam) speech was select and of the highest standard; likewise, it is necessary for the prominent figures of the religion that they become scholars who are eloquent and articulate in their respective languages. In this way, their work of calling (to Islam) will be multiplied. The words of one who uses incorrect expressions and crude speech are neither listened to nor do they have any effect.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2546