The Quran is the word of Allah  (سبحانه وتعالى) the Creator, Sustainer and Originator of the Universe. It was revealed to the Prophet Muhammad (صلى الله عليه وسلم) through Jibreel (عليه السلام) in a auditory manner and the Prophet (صلى الله عليه وسلم) would memorise it from him and convey it his companions. Each year during Ramadan the Prophet (صلى الله عليه وسلم) would review the Quran with Jibreel once apart from his last Ramadan where the Quran was reviewed twice. It is the position of Ahlus Sunnah Wa'l Jam'aah that all the Prophet's were infallible in conveying the revelation. Allah (سبحانه وتعالى‎) Himself promises to preserve the Quran whereas the previous nations were burdened with the responsibility of preserving their revelation. The Quran was revealed bit by bit to Muhammad (صلى الله عليه وسلم) over the course of 23 years, and was written down during his lifetime. It is recorded that more than 15 of the companions had written down most of the Quran but they were not complete, for example, Ibn Mas'ood (رضي الله عنه) had 106 surah's (complete Quran has 114) written down. The Prophet  (صلى الله عليه وسلم) made sure the entire Quran was written during his lifetime however it wasn't compiled into one mushaf until two years after his death during the khilafah of Abu Bakr (رضي الله عنه).