Who was the first to lay down the rules of the Arabic language?

The first person to lay down the rules of Arabic grammar (Nahw) was Abu al-Aswad al-Du'ali, under the guidance of Ali ibn Abi Talib, may Allah be pleased with him. The reason for this was the prevalence of linguistic errors (lahn) in the language after the Arabs intermingled with non-Arabs, and an incident involving his daughter's linguistic mistake, which distressed him. So, he came to Imam Ali, who commanded him to write "In the name of Allah, the Most Gracious, the Most Merciful," and explained to him the parts of speech and something about the chapter on exclamations. Then he told him: "Follow this pattern (unhu 'ala hadha an-nahw) and make your measurements upon it." From this, the rules were named Nahw (grammar).