SB 6.12.23
sri-suka uvaca
iti bruvanav anyonyam
dharma-jijñasaya nrpa
yuyudhate maha-viryav
indra-vrtrau yudham pati
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sri Sukadeva Gosvami said: Vrtrasura and King Indra spoke about devotional service even on the battlefield, and then as a matter of duty they again began fighting. My dear King, both of them were great fighters and were equally powerful.