Shatapatha Brahmana 8.3.4.5

8.3.4.5

atha yāḥ sapta paścāt | sapta vā ime paścātprāṇāścatvāryūrvaṣṭhīvāni dvai pratiṣṭhe yadavāṅnābhestatsaptamamaṅge 'ṅge hi prāṇa ete vai sapta paścātprāṇāstānasminnetaddadhāti

And as to what seven (bricks) he places behind,--there are seven vital airs here in the back part--the four thighs and knee-bones, the two feet, and what is below the navel that is the seventh, for in each limb there is a vital air: this makes seven vital airs here at the back; it is them he thus lays into him.