Śatapatha Brāhmaṇa ("of a hundred paths") 3.6.3.16

3.6.3.16

yadveva vaiṣṇavyarcā juhoti | kanīyāṃsaṃ vā enametadbadhātkṛtvātyanaiṣu stokameva stoko hyaptustametadabhayam prāpya ya evaiṣa taṃ karoti yajñameva yajño hi viṣṇustasmādvaiṣṇavyarcā juhoti

And as to his offering with a verse addressed to Vishnu, it was thus that they made him (Soma) to be too small for the deadly shaft and led him safely across in the form of a drop, for the drop is nimble. And having attained to safety, he now makes him the one he really is, namely, the sacrifice, for Vishnu is the sacrifice: therefore he offers with a verse addressed to Vishnu.