Saphira

 To increase her endurance, Glaedr had her breathe fire for hours upon a natural stone pillar in an attempt to melt it. At first Saphira could only maintain the flames for a few minutes at a time, but before long the blistering torch roared from her maw for over a half hour uninterrupted, heating the pillar white-hot.

Eldest, The Obliterator, pg 596