Agniveshyagrihyasutra 3.9.1.8

3.9.1.8

śirastaḥ prathamaṃ gṛhītvāthorasto 'tha jaṭharato 'thorubāhubhyāmatha patta iti trayastriṃśatamasthīni gṛhṇātīti vijñāyate trayastriṃśat puruṣaḥ iti \

Having gathered first from the head, then from the chest, then from the belly, then from the two thighs and two arms, then from the feet, he gathers thirty-three bones; for it is known [from scripture]: 'Man is thirty-threefold'.