With the exception of Zaidis, Shi'ites believe in the Imamate, a principle by which rulers are Imams who are divinely chosen, infallible and sinless and must come from the "Ahl al-Bayt" regardless of majority opinion, shura or election. They claim that before his death, Muhammad had given many indications, in the hadith of the pond of Khumm in particular, that he considered Ali, his cousin and son-in-law, as his successor. For the Twelvers, Ali and his eleven descendants, the twelve Imams, are believed to have been considered, even before their birth, as the only valid Islamic rulers appointed and decreed by God. Shia Muslims believe that all the Muslim caliphs following Muhammad's death to be illegitimate due to their unjust rule and that Muslims have no obligation to follow them, as the only guidance that was left behind, as ordained in the hadith of the two weighty things, was the Islamic holy book, the Quran and Muhammad's family and offspring, who are believed to be infallible, therefore able to lead society and the Muslim community with complete justice and equity. The Prophet's own grandson, and third Shia Imam, Hussain ibn Ali led an uprising against injustice and the oppressive rule of the Muslim caliph at the time at the Battle of Karbala. Shia Muslims emphasise that values of social justice, and speaking out against oppression and tyranny are not merely moral values, but values essential to a persons religiosity.