Snorri Sturluson wrote The Prose Edda, a telling of Norse mythology. He lived from 1179 – 1241. Here is an excerpt about dwarves.
Next after this, the gods enthroned themselves in their seats and held judgment, and called to mind whence the dwarves had quickened in the mould and underneath in the earth, even as do maggots in flesh. The dwarves had first received shape and life in the flesh of Ymir, and were then maggots; but by decree of the gods had become conscious with the intelligence of men, and had human shape. And nevertheless they dwell in the earth and in stones. Modsognir was the first, and Durinn the second; so it says in Voluspa.
1916 Brodeur translation, 2006 Dover edition, pages 25 and 26.