The Magic Swan is a European fairy tale collected by Hermann Kletke.
Andrew Lang included it in The Green Fairy Book.
[1] Two older brothers abused the youngest son, Peter.
When he did, she told him he should go to a certain tree, where he would find a man asleep and a swan tied to a tree; he should take the swan without waking the man, and everyone would fall in love with its plumage, but when they touched it, he could say "Swan, hold fast" and they would be prisoners.
He collected a great string of people, and the princess laughed at the sight.