Atharvaveda 5.6.4

5.6.4

páry ū ṣú prá dhanvā vā́jasātaye pári vr̥trā́ṇi sakṣáṇiḥ |
dviṣás tád ádhy arṇavéneyase sanisrasó nā́māsi trayodaśó mā́sa índrasya gr̥háḥ ||4||

Round about do thou run forward in order to the winning of booty, round about overpowering adversaries (vṛtrá, n.); then thou goest over haters by the sea (arṇavá). Weakling (sanisrasá) by name art thou, the thirteenth month, Indra's house.