Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.5.5

4.2.5.5

athānyatarattṛṇam | cātvālamabhiprāsyati devānāmutkramaṇamasīti yatra vai devā yajñena svargaṃ lokaṃ samāśnuvata ta etasmāccātvālādūrdhvāḥ svargaṃ lokamupodakrāmaṃstadyajamānamevaitatsvargyam panthānamanusaṃkhyāpayati

He (the Adhvaryu) then throws one of the two stalks of grass forward towards the pit, with, 'Thou art the ascent of the gods!' for when the gods through the sacrifice attained to the heavenly world, it was from that pit that they went upwards to the heavenly world: he thus makes the sacrificer look along the road to heaven.