Sixth Pillar of Islam
Possible Sixth Pillar: Duty to do Good
In his book on Islam, Khaled Abou el Fadl says that in earlier times, many Muslims believed that there were six pillars of Islam, not five. El Fadl says: "The sixth pillar is summed up in the proclamation that every Muslim has a duty to enjoin the good and forbid the evil. Today, all Muslims agree that [this] is a solemn religious duty … but few would still count it as the sixth pillar of Islam." 
Controversy over Sixth Pillar of Islam
Most Sunni Muslims believe there are precisely five Pillars of Islam, since Sunni leaders have taught that there are only five major pillars of the faith. Traditionalists say that no sixth pillar should be added, because changing the pillars would be altering the religion and its beliefs, and so one who believes that there is a sixth is committing a sin. Thus, Sunnis believe that a "six pillar of faith" is outside the folds of mainstream Islam. The sixth pillar of Islam is not included in the Sunni way of life.
- El Fadl, The Great Theft (San Francisco: Harper-Collins, 2005), p. 122.