§ Of Beren and Lúthien The Silmarillion

On an evening of autumn Felagund and Beren set out from Nargothrond with their ten companions; and they journeyed beside Narog to his source in the Falls of Ivrin. Beneath the Shadowy Mountains they came upon a company of Orcs, and slew them all in their camp by night; and they took their gear and their weapons. By the arts of Felagund their own forms and faces were changed into the likeness of Orcs; and thus disguised they came far upon their northward road, and ventured into the western pass, between Ered Wethrin and the highlands of Taur-nu-Fuin. But Sauron in his tower was ware of them, and doubt took him; for they went in haste, and stayed not to report their deeds, as was commanded to all the servants of Morgoth that passed that way. Therefore he sent to waylay them, and bring them before him.