Hyginus Fabulae, 144


     Men previously used to ask for fire from the gods, and they did not know how to store it for use on demand (i.e. "in perpetuum"). But later Prometheus brought it in a fennel stalk from the heavens to earth and showed men how to break it off in some ash and preserve it. Because of this, Mercury received instruction from Jupiter and bound Prometheus with iron fetters to the rock on Mt Caucasus and placed an eagle there to eat out his heart. As much as the eagle ate by day grew back by night. This eagle after 300 years Hercules killed, and he freed Prometheus.
     — translation RTMacfarlane
     

144. Prometheus.
      Homines antea ab immortalibus ignem petebant, neque in perpetuum seruare sciebant; quod postea Prometheus in ferula detulit in terras, hominibusque monstrauit quomodo cinere obrutum seruarent. (2) ob hanc rem Mercurius Iouis iussu deligauit eum in monte Caucaso ad saxum clauis ferreis, et aquilam apposuit quae cor eius exesset; quantum die ederat, tantum nocte crescebat. hanc aquilam post xxx annos Hercules interfecit, eumque liberauit.