The Sunni Muslims believe that the Prophet did not explicitly declare a successor. As for the Shia Muslims, they believe that the Prophet publicly designated his cousin and son-in-law, Hazrat Ali (peace be upon him), as the first in a line of hereditary Imams from the Prophet's family to lead the community after him.
The Sunni Muslims comprise 87–90% of the world's 1.5 billion Muslim population.