Shatapatha Brahmana 1.9.1.25

1.9.1.25

tadvā ṛṣīṇāmanuśrutamāsa | śamyurha vai bārhaspatyo 'ñjasā yajñasya saṃsthāṃ vidāṃ cakāra sa devalokamapīyāyeti te tāmeva yajñasya saṃsthāmupāyanyāṃ śamyurbārhaspatyo 'vedyacamyorabruvaṃstāmvevaiṣa etadyajñasya saṃsthāmupaiti yāṃ śamyurbārhaspatyo 'vedyacamyorāha tasmādvai śamyorāha

It then became known to the Rishis, that Samyu Bârhaspatya had perceived, in its true nature, the consummation of the sacrifice, and had gone to share in the world of the gods. By pronouncing the samyoh, they attained to that same consummation of the sacrifice which Samyu Bârhaspatya had perceived; and to that same consummation of the sacrifice, which Samyu Bârhaspatya had perceived, this (Hotri) attains by pronouncing the sam-yoh. For this reason he pronounces the 'All-hail and blessing.'