Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.10

1.6.3.10

atha yadabravīdindraśatrurvardhasveti | tasmādu hainamindra eva jaghānātha yaddha śaśvadavakṣyadindrasya śatrurvardhasveti śaśvadu ha sa evendramahaniṣyat

And because he (Tvashtri) said, 'Grow thou, having Indra for thy foe!' therefore Indra slew him (Vritra). Had he said, 'Grow thou, the foe (slayer) of Indra!' he (Vritra) would certainly have forthwith slain Indra.