Śatapatha Brāhmaṇa ("of a hundred paths") 1.8.3.16

1.8.3.16

athaikaṃ tṛṇamapagṛhṇāti | yajamāno vai prastaraḥ sa yatkṛtsnam prastaramanupraharetkṣipre ha yajamāno 'muṃ lokamiyāttatho ha yajamāno jyogjīvati yāvadvevāsyeha mānuṣamāyustasmā evaitadapagṛhṇāti

He then takes a single stalk from it. The prastara-bunch is the sacrificer; and therefore, if he were to throw the whole prastara (at once) into the fire, the sacrificer would speedily go to yonder world. In this way, however, the sacrificer will live long; and what the full measure of human life here on earth is, for that he takes this (single stalk) therefrom.