Shatapatha Brahmana 6.5.3.11

6.5.3.11

saptāśvaśakāni bhavanti | spta yajūṃṣi saptatayya etā devatāḥ sapta śīrṣanprāṇā yadu vā api bahukṛtvaḥ sapta-sapta saptaiva tacīrṣaṇyeva tatsapta prāṇāndadhāti

Seven balls of horse-dung are (used), and seven formulas: those deities are sevenfold, and seven vital airs there are in the head. But also what is many times, seven times seven, is (expressed by) seven: he thus puts the seven vital airs into the head.