Imamah is the divine leadership after the Prophet (SAWA) to lead and guide Muslims to the real teachings of Islam. It was mentioned in hundreds of times by the Prophet (SAWA) including the greatest announcement in Islam on the Day of Ghadeer when the Prophet ordered all Muslims to give allegiances to Ali as the commander of the faithful.
Muslims are required to obey the Prophet (SAWA) as Allah (SWT) says in Quran : Who obeys the Prophet has in fact obeyed Allah.(Sura Annisaa', verse 80).
There are hundreds of authentic narrations in Sunni books about this most authentic prophetic order