How are the teachings of Muhammad preserved?

Answer 1

According to Islamic Theology, the Qur'an was transmitted by the Archangel Gabriel from God to Muhammad. Muhammad's recitation was memorized by faithful Muslims and protected by God, to make sure that the Qur'an never change. As concerns, Muhammad's actual teachings, those are the Hadiths (Events) and the Sirat an-Nabi (Biography of the Prophet). The Hadiths have numerous origins and while some are legitimate, many track back to local cultural traditions, other religions' holy texts or prayers, and personal beliefs of local leaders. Islamic scholars sorted out the Hadiths and determined their accuracy by using isnads (chains of retellers) that would track back to one of Muhammad's Sahaba (Apostles). Those that passed the test were preserved in Hadith Collections. The Biography of the Prophet is a document written by Ibn Ishaq and redacted by Ibn Hisham roughly two centuries after Muhammad's death based on the stories, legends, and oral traditions surrounding Muhammad.

Secular History does not confirm any of the claims about the Qur'an or the Hadiths. There is still no coherent hypothesis of the origin of the Qur'an (as opposed to the Pentateuch's JEPD hypothesis), but scholars generally believe that the Qur'an was written by several groups of writers, editors, and redactors in the 7th and 8th centuries. On Hadiths, scholars like Goldziher have demonstrated that the isnad method of determining the accuracy of the Hadiths was misplaced and a number of Hadiths changed their isnads over time, meaning the tradition preserved in the Hadith Collections is not likely to be an accurate demonstration of what Muhammad did. It is generally agreed that the Sirat an-Nabi was composed from latent legends in the 9th century, but Secular Historians reject that these are in any way accurate about Muhammad, even assuming he did live. (The details of any historical event mutate within a few decades after the event, especially in a primarily illiterate culture, which the Arab Muslims were.)

Answer 2

Muhammad's teachings were preserved in the Qur'an ad the Hadith as sacred writings.

Answer 3

The Qur'an was preserved in his life time. The teachings and sayings of Hazrat Muhammad (SAW) have been very carefully preserved in the books of Hadith by the Muslim scholars.