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.
Lúthien was spent, and she had not time nor strength to quell the wolf. But Beren strode forth before her, and in his right hand he held aloft the Silmaril. Carcharoth halted, and for a moment was afraid. “Get you gone, and fly!” cried Beren; “for here is afire that shall consume you, and all evil things.” And he thrust the Silmaril before the eyes of the wolf.
Source: J. R. R. Tolkien’s “The Silmarillion”, Chapter 19: Of Beren and Lúthien