Were the rules of Tajwid (Al-Idgham, Al-Ikhfa, and Al-Iqlab) present in the Arabic language before Islam, or were they taught to the Prophet—peace and blessings be upon him—by Gabriel?

The rules of Tajwid were present in the Arabic language, and the Quran was revealed with them. The Prophet's (PBUH) saying: "Recite the Quran with the melodies and voices of the Arabs," indicates this. The Quran was revealed in the language of the Arabs; therefore, the rules of Tajwid, by which we recite it, were present in their language. Gabriel used to recite it to the Prophet (PBUH) with these rules, and the Prophet would recite it just as Gabriel had recited it, as Allah Almighty said: "So when We recite it, then follow its recitation" [Al-Qiyamah:18]. And thus, the Prophet would recite it to his companions.