Three sources of Islam studies

First, al-Qur'an

From Wiki: The Qur’an^[1] (Arabic: القرآن‎ al-qur’ān, literally “the recitation”; also sometimes transliterated as Quran, Qur’ān, Koran, Alcoran or Al-Qur’ān) is the central religious text of Islam. Muslims believe the Qur’an to be the book of divine guidance and direction for mankind, and consider the original Arabic text to be the final revelation of God.^[2][3][4][5]

Qur'an is the holly book of Islam, including the first edition to the latest edition, including every words the God said and angle Gabriel(גבריאל） delivered to humans, which can't be wrong and can't be changed. People should never challenge the order of God.
However, Qur'an is not a reliable source of history of Islam.

Second, Tafsir

From Wiki: Tafsir (Arabic: تفسير‎, tafsir, "interpretation") is the Arabic word for exegesis or commentary, usually of the Qur'an. It does not include esoteric or mystical interpretations, which are covered by the related word Ta'wil. An author of tafsir is a mufassir (Arabic: 'مُفسر‎, mufassir, plural: Arabic: مفسرون‎, mufassirūn).

Third, Sirah

From Wiki: Sīrah Rasūl Allāh (Life of the Messenger of God; Arabic: سيرة رسول الله‎) or Sīrat Nabawiyya (Life of the Prophet; Arabic: سيرة نبوية‎) (from Arabic سيرة) is the Arabic term used for the various traditional Muslim biographies of Muhammad, from which most historical information about his life and the early period of Islam is derived. The name is often shortened to "Sīra" or "Sīrah" ("life" or "journey"; Arabic: سيرة‎).

Every Muslim should obey what the prophet said, why? Because it is not him who saying the words, it is God. Prophet just delivered the words from God.