The Völsunga saga is a legendary saga based on the origin and decline of the Völsung clan. When Tolkien was a student, the only way of reading it in English was through the translation of 1870 made by William Morris and Eirikur Magnusson. 

The most relevant events in the Völsunga saga are the quest led by Sigmund and Sinfjötli to save princess Signy from the king Siggeir, Sigurd killing the serpent/dragon Fafnir; and the influence of the cursed ring Andvaranaut. The similarities with Tolkien works can be seen in the figure of dragon (Fafnir vs Smaug) and the cursed ring Andvaranaut, which resembles the One Ring (both are cursed and magical golden ring). Moreover, in the saga we have a  broken sword reforged called Gram. Also in The Lord of the rings there is a broken sword reforged, Andúril.