SB 6.12.23

śrī-śuka uvāca
iti bruvāṇāv anyonyaṁ
 dharma-jijñāsayā nṛpa
yuyudhāte mahā-vīryāv
 indra-vṛtrau yudhām patī
Word for word: 
śrī-śukaḥ uvāca — Śrī Śukadeva Gosvāmī said; iti — thus; bruvāṇau — speaking; anyonyam — to one another; dharma-jijñāsayā — with a desire to know the supreme, ultimate religious principle (devotional service); nṛpa — O King; yuyudhāte — fought; mahā-vīryau — both very powerful; indra — King Indra; vṛtrau — and Vṛtrāsura; yudhām patī — both great military commanders.
Translation: 
Śrī Śukadeva Gosvāmī said: Vṛtrāsura 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.