Titus Livius (Livy), The History of Rome, Book 26
Frank Gardner Moore, Professor Emeritus in Columbia University, Ed.

7. but Hannibal, seeing that it was impossible either to tempt the enemy into battle again, or to break a way through their camp to Capua, for fear the new [2??] consuls should cut off his supplies also, resolved to retire without accomplishing his undertaking and to move his camp away from Capua. [3] while carefully considering whither he should remove, the impulse came to him to proceed to Rome, the very centre of the war. it was something which he had always desired to do, but after the battle of Cannae he had let the opportunity pass, as others commonly complained, and as he himself frequently admitted. [4] in unexpected alarm and confusion it need not be beyond his hopes that some part of the city could be seized. and if Rome should be in danger, he thought that either both of the Roman commanders or one of them would at once abandon Capua; and that if they should divide their forces, each being weakened would give either himself or the Capuans the chance of success. [5] only one concern tormented him, the fear that as soon as he had withdrawn, the Capuans might at once be surrendered. [6] a Numidian who was ready to dare and do anything was induced [p. 29]by Hannibal's gifts to take a letter, enter the Roman¹ camp under the guise of a deserter, and then from the other side of the camp make his way in secret to Capua. [7] and the letter was filled with encouragements. his departure, Hannibal said, which would be of advantage to them, would draw off the Roman generals and armies from the siege of Capua to the defence of Rome. [8] they should not be downcast; by holding out for a few days they would cause the entire blockade to be raised. [9] he then ordered that boats on the Volturnus should be seized and rowed up to the fort which he had previously built for a defence.² [10] and when word came that the number of these was such that his army could be ferried across in a single night, he had food prepared for ten days, led his legions down to the river by night, and transported them across before daylight.