§ Of Beren and Lúthien The Silmarillion

Then terror fell upon Beren and Lúthien, and they fled, heedless and without disguise, desiring only to see the light once more. They were neither hindered nor pursued, but the Gate was held against their going out; for Carcharoth had arisen from sleep, and stood now in wrath upon the threshold of Angband. Before they were aware of him, he saw them, and sprang upon them as they ran.