In Destiny's Clutch by Rafael Sabatini

Trembling still from his recent emotion, Brancaleone hoarsely bade the corsair call up his officers and repeat his words before them. "And you shall make oath upon this matter," he added. "Men say of you that you are a faithful Moslem. I mean to put it to the test." Dragut, now all eagerness to know what plan was stirring in his prisoner's brain, unable to brook further suspense in this affair, called up his officers, and before them all, taking Allah to witness, he made oath upon the beard of the Prophet that if Brancaleone could show him deliverance, he on his side would recompense the Genoese to the extent demanded.

Thereafter Dragut and Brancaleone went ashore, with no other attendant but the Nubian swordsman. It was the Genoese who led the way, not toward the fort, as Dragut had expected, but in the opposite direction. Arrived at the northernmost curve of that almost circular lagoon, where the ground was swampy. Brancaleone paused. He pointed across a strip of shallow land, that was no more than a half mile or so in width, to the blue-green sea beyond. Part of this territory was swamp, and part sand; vegetation there was of the scantiest; some clumps of reeds, an odd date palm, its crest rustling slightly in the breeze, and nothing else.