Illiad

Stanza 80
Achilles asks the seers why Apollo is angry with them.
Stanza 90
Calchas said he would tell Achilles the answer if he swears to protect Calchas whenever he tries to talk to the king, Agamemnon.
Stanza 100
Achilles agrees and swears by the god Apollo that he will never let a soldier or Agamemnon lay a hand on Calchas.
Stanza 110
Calchas says that Apollo is angry with Agamemnon because he burned his priest so the priest prayed to the god and Apollo answered his prayer to keep the plague going on until they carry out a task.