Sunnah
Sunnah or Sunna (سنة, plural سنن sunan), is an Arabic word that means "habit" "way" or "usual practice". Within the context of Islam it refers to the words and actions or example of Prophet Muhammad, who is known to them as al-Insān al-Kāmil (the perfect human) and uswa hasana (an excellent model of conduct).
The Qur'an instructs Muslims to obey the Messenger, and to do this they have to refer to the Sira and Hadith which collectively form Muhammad's Sunnah.
Even the most insignificant of actions on his part (found therein) have a drastic effect upon Islamic doctrines and laws, as the Qur'an and Sunnah collectively form the Shari'ah.
Bid'ah (innovation) is forbidden, and the closer one emulates Muhammad's behavior the more pious they are considered.