Iliad
Who told them how to get rid of their problem with Apollo?
Chapter 1
Chapter 1
Calchas tells them that the plague has been sent by Apollo in punishment for Agamemnon's treatment of Chryses. To atone for the sin, the Achaeans must give Chryseis back without accepting any ransom and in addition they must give a hundred sacred bulls to Chryses for sacrifice.
http://www.gradesaver.com/iliad/study-guide/summary-books-1-4