And you would think them awake, while they were asleep. And We turned them to the right and to the left, while their dog stretched his forelegs at the entrance. If you had looked at them, you would have turned from them in flight and been filled by them with terror. [18:18]
From Tafsir,
(and their dog stretching forth his two forelegs at the Wasid)
Ibn `Abbas, Mujahid, Sa`id bin Jubayr and Qatadah said: "The Wasid means the threshold.'' Ibn `Abbas said: "By the door.'' It was said: "On the ground.'' The correct view is that it means on the threshold, i.e., at the door.
Their dog lay down at the door, as is the habit of dogs. Ibn Jurayj said, "He was guarding the door for them.'' It was his nature and habit to lie down at their door as if guarding them. He was sitting outside the door, because the angels do not enter a house in which there is a dog, as was reported in As-Sahih, nor do they enter a house in which there is an image, a person in a state of ritual impurity or a disbeliever, as was narrated in the Hasan Hadith. The blessing they enjoyed extended to their dog, so the sleep that overtook them overtook him too. This is the benefit of accompanying good people, and so this dog attained fame and stature.
Source:
http://www.qtafsir.com/index.php?opt...2738&Itemid=73