How Hadeeth has been preserved till now??? (part3)

"O ye who believe! Obey Allah and obey the Prophet and those charged with authority among you. Therefore, if there is a difference of opinion among you in any matter, refer it back to Allah and His Prophet, if ye do believe in Allah and the Last Day." (4:5)

This verse of the Quran clearly indicates that the original sources of knowledge on Islam are only two: The Holy Quran and the Sunnah of the Prophet (PBUH)

SO how the saying (hadith) of the prophet Peace and Blessings Upon Him has been preserved till now?
you will see that in this video (part3)