The followers of the Holy Prophet ﷺlearnt by heart and started following upon them.
Besides these four Holy Books, other small Sahi-fas were also revealed on other prophets. This is the most sacred book among the three others, Torat, Zabur and the Bible. After the declaration of His prophet hood at the age of 40. The Holy Quran was revealed on the Holy Prophet ﷺ in 23 years. An Introduction to the Holy Quran and its Characteristics