In 656, Caliph Uthman, the third caliph to rule after the death of the Prophet, was assassinated at his home in Medina (now Saudi Arabia) by besieging Muslim rebels from Mesopotamia.
The new sultan will be chosen from Sokoto's ruling dynasty - descendents of Uthman Dan Fodio, who launched a jihad, or holy war, in 1804, spreading Islam across northern Nigeria and its neighbours.
In this letter bin Laden mentioned by name a number of his children living in Iran including his sons Ladin, Uthman and Muhammad and his daughter Fatima, who is married to Sulaiman Abu Ghaith who now sits in a Manhattan jail.