Just as we know stars and planets existed, but terms like this is called a ‘star’, this is defined as a ‘planet’ etc. is new. The matter was always there. Similarly, flowers and plants were there, but the science gives terms and classifications.
We know Tajweed existed because the Qur’ân was revealed with Tajweed. There is a famous hadith about ibn Mas’ood and a Surah Tawbah ayah.
A man was reciting the Qur’ân to Abdullâh ibn Mas’ood and he recited,“InnamasSadaqâtu lil-Fuqara-i wal-Masâkeen, Ibn Mas’ud said, “This is not how the messenger of Allâh recited it to me!”, so the man inquired, “how did he recite it to you?” so he said, “lil-Fuqarââ-i wal-Masâkeen”, he prolonged the vowel. This is a proof that the study of Tajweed was taught by the prophet sallAllâhu alayhi wa sallam and the Sahâbah were very particular. Even though the madd at the time was not called Madd Muttasil but practically, the rules were applied.
In the beginning of Islâm, the Arabs knew the language very well. It was not written as it is written today. There were no harakât and no dots. The Arabs knew how to read it because they knew the language very well. When Islâm spread to non-Arabs, they were not able to read in the same way. Later the scholars started adding signs for irâbs.
The Sahâbah were particular about grammar.
’Ali radhiAllâhi anhu at his time asked a famous scholar by name Abul Aswad al Du‘ali to add signs as people were losing the grammar.
There was also a narration from ’Umar. Once a youth replied back to something he said with wrong grammar. ’Umar said your grammar mistake made me more angry than the error we were discussing.
One day, Abul Aswad and his daughter were looking at the sky. She said (in Arabic):
مَا أَجْمَلُ السَّمَاءِ
This meant, what is the most beautiful thing in the sky? He replied, the stars. She said no I meant the sky is beautiful. So he said if you wanted to say it, you should have said it this way:
مَا أَجْمَلَ السَّمَاءَ
So the ending of the sentence changed the meaning.
So he realized he needed to document Arabic grammar. He hired someone and told him that I have this Qur’ân in front of you. I will recite and when I make an oo sound, put this sign before the letter. When you see me open my mouth, put a sign on top of the letter. And when you see ee then put a sign below. Later the harakât changed. The dhammah and fathah were above and kasrah beneath the letter.
When non-Arabs came into Islâm, there were letters not present in other languages. So they felt the need to document articulation points of letters. They would write where letters were articulated from, e.g. ba from the 2 lips. However, there was another problem. If a language has p and no b, then when they were only told where to articulate from, then instead of b they would say p. So scholars realized you needed something besides the articulation point. This is how sifât developed.
Some of the famous grammar scholars like Seebaway, Al Khaleel.
What we know as rules nowadays were documented in the same books and were all part of the same thing. It was only later that the science of Tajweed as a separate science that developed. In the 4th century Abu Mazâhem al Khâqâni started a separate science.
People wrote about different aspects of Tajweed in the 2nd century but the subject started in the 4th century. Abu Amr, one of the 10 qurra wrote an article about Idghâm Kabeer in the 2nd century. It is called Risâla fi Idghâm Kabeer. So individual topics were written about but not Tajweed as a separate science.
Language scholars, when documenting the language named the terms like izhâr, etc. Older Arabic grammar books would include rules of Tajweed. These were documented as part of grammar or Tajweed. Only in the 4th century, rules related to reciting were taught as a separate science. This was started by Abu Mazâhem al Khâqâni.
The rules of recitation were taken as the science of Tajweed. But to master stopping and starting you have to know the rules of Arabic grammar.
Initially everything was orally passed down. After a certain period of time, students were requesting things to be written and that’s when poems were written. Writing books and poems came later. So then we see Imâm Shâtibiyy wrote about the 7 ways of reciting.
The poem by Khâqâni is the first piece of writing that spoke about the science of Tajweed in a general way. He was the first to single out Tajweed and write about it. Everything was already being taught and passed down.
The word Tajweed was not used. The word used was Husn ul Ada (good/ excellent performance).
Even though the word Tajweed was being used, it was not common. Abu Amr ad Dâni narrated that Ibn Mujâhid said there are 2 types of lahn, and he mentioned the word Tajweed. This proves the word Tajweed was used but not common.
 The question arises, is the last harakah not converted to a sukoon for stopping? Ideally Arabic should be spoken like the Qur’ân is recited, that is, the same rules, so you would only skip the harakah of the last letter you stopped on (due to stopping).
Nowadays Arabs don’t say the last harakas of any words in the sentence (generally) because:
1. Arabs don’t know their own grammar
2. Modern dialects have polluted the original language so much that it’s now very different to the original Arabic dialects spoken at the time of the Prophet صلى الله عليه وسلم
Even then, some professional people who like to show their eloquence will continue to still speak Arabic with the endings of the words in formal situations, such as some newsreaders etc. (That is if they know their grammar)