Sertorius (13.5-13.6)

But Sertorius quickly came to the rescue and ordered two thousand skins to be filled with water, offering for each skin a considerable sum of money. Many Iberians and many Maurusians volunteered for the work, and after selecting men who were both sturdy and swift of foot, he sent them by a route through the mountains, with orders that when they had delivered the skins to the people in the city, they should secretly convey away the unserviceable mass of the population, in order that the water might suffice for the actual defenders of the city.

When Metellus learned that this had been done, he was annoyed, since his soldiers had already consumed their provisions, and sent out Aquinus, at the head of six thousand men, to forage. But Sertorius learned of this and set an ambush of three thousand men in the road by which Aquinus was to return. These sallied forth from a shady ravine and attacked Aquinus in the rear, while Sertorius himself assailed him in front, routed him, slew some of his men, and took some of them prisoners. Aquinus, after losing both his armour and his horse, got back to Metellus, who then retired disgracefully, much flouted by the Iberians.