Shatapatha Brahmana 8.5.4.11

8.5.4.11

tā u vai pañcaiva | yajuṣānyāsu purīṣaṃ nivapati tūṣṇīmatra tenaiṣā na citiratho lokampṛṇāntā anyāścitayo bhavanti nātra lokanpṛṇāmupadhāti teno evaiṣā na citiḥ

And, indeed, there are only five of them,--on the other (layers) he throws down the loose soil with a prayer, and here (he does so) silently: in that respect this is not a layer. And the other layers end with space-fillers, but here he lays down no space-filler: in that respect also this is not a layer.