Who was the first to add diacritical marks to the Mushaf?

Allah Almighty says: "Indeed, We have sent down the Dhikr (the Qur’an), and indeed, We will be its Guardian" (Al-Hijr: 9). And part of His preservation of His Book is that He prepared for it memorizers and skilled scribes. The Messenger of Allah would command the placement of verses in their proper positions, and there were no dots or diacritical marks on the letters at that time. When errors (lahn) in the language became widespread after non-Arabs entered Islam, Abu al-Aswad al-Du'ali complained to Ali ibn Abi Talib about the numerous mistakes in reciting the Qur'an. Ali then taught him the principles of grammar. Abu al-Aswad al-Du'ali placed the dots for vocalization (tashkeel): he would place a dot in front of the letter for a dammah, above it for a fathah, and below it for a kasrah. As for Al-Khalil ibn Ahmad al-Farahidi, he introduced a more precise vocalization system using a horizontally stretched alif for fathah and kasrah, and the head of a waw for dammah. Then he placed the dots for distinguishing (i'jam) letters. In summary, Abu al-Aswad al-Du'ali was the first to introduce the dots for vocalization (tashkeel), while Al-Khalil ibn Ahmad was the first to introduce the dots for distinguishing letters (i'jam).