The Quran is the literal word of God, which He revealed to His Prophet Muhammad (peace of God be upon him) through the Angel Gabriel. The Quran is the ultimate source of authority because it’s not only revelation or inspiration of God, it’s verbatim word or revelation of God. It was memorized by Prophet Muhammad (peace of God be upon him) who then dictated it to his Companions. They, in turn, memorized it, wrote it down, and reviewed it with the Prophet Muhammad (peace of God be upon him) . Moreover, the Prophet Muhammad (peace of God be upon him) reviewed the Quran with the Angel Gabriel once each year and twice in the last year of his life. From the time the Quran was revealed, until this day, there has always been a huge number of Muslims who have memorized all of the Quran, letter by letter. Some of them have even been able to memorize all of the Quran by the age of ten. Not one letter of the Quran has been changed over the centuries.
Allah says in the Quran (which is translated to English as). Chapter 10, verses 57-58. O mankind, there has to come to you instruction from your Lord and healing for what is in the breasts and guidance and mercy for the believers.
Listen to Quran with translation
The Opening chapter in the Quran
Interpretation of the Opening Chapter in the Quran
Sunnah
The sunnah (what the Prophet Muhammad 
said, did, or approved of) is the second source in Islam. The sunnah is comprised of hadeeths, which are reliably transmitted reports by the Prophet Muhammad’s companions of what he said, did, or approved of. Belief in the sunnah is a basic Islamic belief.
Sahih al-Bukhari
Sahih al-Bukhari is a collection of hadith compiled by Imam Muhammad al-Bukhari (d. 256 AH/870 AD) (rahimahullah). His collection is recognized by the overwhelming majority of the Muslim world to be the most authentic collection of reports of the Sunnah of the Prophet Muhammad .
Sahih Muslim
, and along with Sahih al-Bukhari forms the “Sahihain,” or the “Two Sahihs.”