Where does this quote come from?

[Inziladun]

Off hand, that doesn't sound like something I've read in Tolkien's books.

It does remind me, however, of a description of Galadriel from UT in the chapter The History of Galadriel and Celeborn.

Even among the Eldar [Galadriel] was accounted beautiful, and her hair was held a marvel unmatched. It was golden like the hair of her father and of her foremother Indis, but richer and more radiant, for its gold was touched by some memory of the starlike silver of her mother; and the Eldar said that the light of the Two Trees....had been snared in its tresses.

I'm unaware of Galadriel ever being called the "fairest" in the books, either. In fact, The Silmarillion makes a point of saying many times over that Lúthien indeed was the most beautiful of all the Children of Ilúvatar.