After the Prophet (PBUH) four caliphs led the Muslims: Abu Bakr (632-634), Umar ibn al-Khattab (634-644), Uthman ibn Affan (644-656) and Ali ibn Abi Talib (656-661). Abu Bakr fought the apostasy wars and began collecting the Qur'an. Umar expanded the state and set up the Hijri calendar. Uthman standardised the Qur'an. Ali was cousin and son-in-law of the Prophet.