Shatapatha Brahmana 6.2.3.9

6.2.3.9

te yadabruvan | cetayadhvamiti citimicateti vāva tadabruvanyaccetayamānā apaśyaṃstasmāccitayaḥ

Now when they said, 'Meditate ye (ketayadhvam)!' they doubtless meant to say, 'Seek ye a layer (kitim ikkhata)!' and inasmuch as meditating (ketay) they saw them, therefore they are 'layers' (kitayah).