 No contemporary Arab source mentions the visit.One detail, added by Bonaventure in the official life of Francis (written forty years after the event), has Francis offering to challenge the Sultan's "priests" to trial-by-fire in order to prove the veracity of the Christian Gospel.Such an incident is alluded to in a scene in the late 13th-century fresco cycle, attributed to Giotto, in the upper basilica at Assisi.