Śatapatha Brāhmaṇa ("of a hundred paths") 1.2.4.3

1.2.4.3

sa yatsphyamādatte | yathaiva tadindro vṛtrāya vajramudayacadevamevaiṣa etam pāpmane dviṣate bhrātṛvyāya vajramudyacati tasmādvai sphyamādatte

Now when he takes up the wooden sword, he raises that thunderbolt against the wicked, spiteful enemy, even as Indra at that time raised the thunderbolt against Vritra: that is the reason why he takes the wooden sword.