SB 9.15.38
rama rama mahabaho
bhavan papam akarasit
avadhin naradevam yat
sarva-devamayam vrtha
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O great hero, my dear son Parasurama, you have unnecessarily killed the king, who is supposed to be the embodiment of all the demigods. Thus you have committed a sin.