Atharvaveda 2.5.5

2.5.5

índrasya nú prá vocaṃ vīryā̀ṇi yā́ni cakā́ra prathamā́ni vajrī́ |
áhann áhim ánu apás tatarda prá vakṣáṇā abhinat párvatānām ||5||

Now will I proclaim the heroisms of Indra, which first he of the thunderbolt (vajrín) did; he slew the dragon (áhi); he penetrated to the waters; he split ⌊forth⌋ the bellies (vakṣáṇā) of the mountains.