لَقَدْ كَانَ فِي قَصَصِهِمْ عِبْرَةٌ لِّأُوْلِي الأَلْبَابِ
Indeed in their stories, there is a lesson for men of understanding. (12:111)
There are a great many examples throughout Islamic classical literature in which the scholars, leaders, and pious men of the past urged and encouraged the Muslims to learn, speak and master classical Arabic, and avoid grammatical mistakes in their speech as much as they were able. The primary reason that drove the scholars of the past to systemize the rules of grammar was the grammatical solecisms (lahn لحن) that were beginning to creep into the tongue of the Arabs, due to the expansion of their borders which led them to mix with non-Arabs and be influenced by their language [among other reasons] and there was a fear that this would lead to an increase in making mistakes when reciting the Qur’an, as had happened in a number of previous cases.
Thus, very early on in the history of Islam we find such examples of encouraging the mastery of Arabic, among which are:
A man went to Ziyad ibn Abeehi and complained to him that his father had died and his brother had taken all the inheritence unlawfully, but made a grammatical mistake in his complaint. Ziyad replied, “The loss you have caused your soul is greater than what you have lost in your wealth.” 
It is reported that ‘Umar ibn Yazeed wrote to Abu Moosa al-Ash’ari (may Allaah be pleased with him) and said: ‘Learn the Sunnah and learn Arabic; learn the Qur’aan in Arabic for it is Arabic.’ 
According to another hadeeth narrated from ‘Umar (may Allaah be pleased with him), he said: ‘Learn Arabic for it is part of your religion, and learn how the estate of the deceased should be divided (faraa’id) for these are part of your religion.’ 
This trend continued throughout the ages, and with the expansions of the Umayyad dynasty in the 7th Century C.E., solecisms became widespread such that they even afflicted the caliphs and leaders such as ‘Abd al-Malik ibn Marwan and al-Hajjaj ibn Yusuf al-Thaqafi. Language became a measure of status such that a man’s social standing would drop were he found to commit solecisms, to the extent that ‘Abd al-Malik ibn Marwan was once told that he his hair had become gray very early, to which he replied, “It is due to my fear of ascending the pulpit and commiting a solecism during my sermon!“ He used to view solecisms in speech to be worse than ripping apart an expensive and precious garment. 
Men were often rewarded greatly for merely being able to speak fluently without mistakes, even if they were undeserving of the reward. For example, the Caliph ‘Umar ibn ‘Abd al-‘Azeez used to say, “A man may come to me asking me for something he deserves, but if he commits a grammatical error while asking I deny him it, for it makes me feel as though I am nibbling at a peice of sour pomegranate due to my anger at hearing his mistake. Similarly, a man may come to me asking for something he does not deserve, but if he says it with correct speech I grant him it, due to my delighting at the speech I hear from him.“ 
These are but a handful of examples of this nature; the books of classical literature are replete with much more of the same.
It is often said that one of the main benefits of studying history is to learn from the past. May Allaah grant us the insight and wisdom to take heed of what our predecessors urged. Ameen.
 ‘Uyoon al-Akhbaar 2/159
 and  Iqtidaa’ al-Siraat al-Mustaqeem, 2/207
 Tarikh Dimishq
 Uyoon al-Akhbaar 2/158
 al-Addaar p245