[The Squatter of Amon Rûdh]

I think that you might have this quotation in mind:

The next day at the hour of sunset Aragorn walked alone in the woods, and his heart was high within him; and he sang, for he was full of hope and the world was fair. And suddenly even as he sang he saw a maiden walking on a greensward among the white stems of the birches; and he halted amazed, thinking that he had strayed into a dream, or else that he had received the gift of the Elf-minstrels, who can make the things of which they sing appear before the eyes of those that listen.

But it's worth continuing the narrative for these lines, which are also worthy of a place on this thread:

For Aragorn had been singing a part of the Lay of Lúthien which tells of the meeting of Lúthien and Beren in the forest of Neldoreth. And behold! there Lúthien walked before his eyes in Rivendell, clad in a mantle of silver and blue, fair as the twilight in Elven-home; her dark hair strayed in a sudden wind, and her brows were bound with gems like stars.

For a moment Aragorn gazed in silence, but fearing that she would pass away and never be seen again, he called to her crying, Tinúviel, Tinúviel! even as Beren had done in the Elder Days long ago.