The 1,400-year-old schism between Sunni and Shia Muslims has rarely been as toxic as it is today, feeding wars and communal strife in Syria, Iraq, Yemen, Pakistan, Afghanistan and elsewhere. But what lies behind the Sunni-Shia split? All Muslims agree on the text of the Qur’an as the word of God – but they also ask who to trust as the transmitters of Muhammad’s custom or sunna, which is vital to the practice of their faith.

The divide between Muslims may go back to the lifetime of the Prophet, or at least to his final hours. Different accounts of who held and comforted Muhammad as he died have survived. Ayesha, the closest of his wives at the end of his life, tells us it was her. But Abdullah bin Abbas, the Prophet’s cousin (and ancestor of the Abbasid caliphs), insists it was Ali, another cousin, who was married to the Prophet’s daughter, Fatima, and was the father of his grandchildren. We cannot know which account is correct.