From Book 17 of the Odyssey:
As they were speaking, a dog that had been lying asleep raised his head and pricked up his ears. This was Argos, whom Odysseus had bred before setting out for Troy, but he had never had any enjoyment from him. In... the old days he used to be taken out by the young men when they went hunting wild goats, or deer, or hares, but now that his master was gone he was lying neglected on the heaps of mule and cow dung that lay in front of the stable doors till the men should come and draw it away to manure the great close; and he was full of fleas. As soon as he saw Odysseus standing there, he dropped his ears and wagged his tail, but he could not get close up to his master. When Odysseus saw the dog on the other side of the yard, dashed a tear from his eyes without Eumaeus seeing it, and said:
"Eumaeus, what a noble hound that is over yonder on the manure heap: his build is splendid; is he as fine a fellow as he looks, or is he only one of those dogs that come begging about a table, and are kept merely for show?"
"This dog," answered Eumaeus, "belonged to him who has died in a far country. If he were what he was when Odysseus left for Troy, he would soon show you what he could do. There was not a wild beast in the forest that could get away from him when he was once on its tracks. But now he has fallen on evil times, for his master is dead and gone, and the women take no care of him. Servants never do their work when their master's hand is no longer over them, for Zeus takes half the goodness out of a man when he makes a slave of him."
So saying he entered the well-built mansion, and made straight for the riotous pretenders in the hall. But Argos passed into the darkness of death, now that he had seen his master once more after twenty years