Shatapatha Brahmana 9.3.1.21

9.3.1.21

atha yaṃ dvitīyamuttararo juhoti | sapta ṛṣayaḥ sa sa saptakapālo bhavati sapta hi sapta 'rṣayastamanantarhitam pūrvasmājjuhotyṛtuṣu tadṛṣīnpratiṣṭhāpayati

And the second (cake) which he offers on the left side, is the seven Rishis; it is one of seven potsherds, for the seven Rishis are seven in number. He offers it close to the preceding one: he thus establishes the seven Rishis in the seasons.