Atharvaveda 9.9.16

9.9.16

sākaṃjā́nāṃ saptátham āhur ekajáṃ ṣáḍ íd yamā́ ŕ̥ṣayo devajā́ íti |
téṣām iṣṭā́ni víhitāni dhāmaśá sthātré rejante víkr̥tāni rūpaśáḥ ||16||

Of those born together the seventh they call sole-born; six, they say, are twins, god-born seers; the sacrifices (? iṣṭá) of them, distributed according to their abodes, quake in their station, being altered (vi-kṛ) in respect to form.